The probability of Bus A arriving before Bus B












6












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Bus A arrives at a random time between 2pm and 4pm, and Bus B arrives at a random time between 3pm and 5pm. What are the odds that Bus A arrives before Bus B?



My understanding is that since Bus B cannot possibly arrive between 2 and 3, we can only talk about the time between 3 and 4 pm, when there is an equal probability for both buses arriving. But in this case, the probability of Bus A arriving before B is 50%. No? What am I missing here? Or I should look at the entire timeline, 2 pm - 5 pm? But then in this case, it is still 50%. Where is my thinking wrong?










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  • 1




    $begingroup$
    Using conditional probability I think it must be. $P(A/B')$ proceeding from here gives us ans $1/22$ is it correct?
    $endgroup$
    – Vimath
    2 days ago










  • $begingroup$
    Yes, the arrival of buses is independent events.
    $endgroup$
    – IrinaS
    2 days ago










  • $begingroup$
    As a nitpick, the words "probability" and "odds" are not interchangeable. They are related, yes, but they do not mean the same thing. The probability of picking an ace from a well shuffled standard deck is $frac{1}{13}$. The odds however are $1:12$ for, or equivalently $12:1$ against. If you only ever want to talk about probabilities, then only use the word probability and avoid using the word odds. Also, @Vimath conditional probabilities are written with a vertical bar, not a slanted bar. It should be $P(Amid B')$, not $P(A/B')$
    $endgroup$
    – JMoravitz
    2 days ago










  • $begingroup$
    @JMoravitz I missed the syntax there , I'll try not to repeat it.
    $endgroup$
    – Vimath
    2 days ago
















6












$begingroup$


Bus A arrives at a random time between 2pm and 4pm, and Bus B arrives at a random time between 3pm and 5pm. What are the odds that Bus A arrives before Bus B?



My understanding is that since Bus B cannot possibly arrive between 2 and 3, we can only talk about the time between 3 and 4 pm, when there is an equal probability for both buses arriving. But in this case, the probability of Bus A arriving before B is 50%. No? What am I missing here? Or I should look at the entire timeline, 2 pm - 5 pm? But then in this case, it is still 50%. Where is my thinking wrong?










share|cite|improve this question









New contributor




IrinaS is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.







$endgroup$








  • 1




    $begingroup$
    Using conditional probability I think it must be. $P(A/B')$ proceeding from here gives us ans $1/22$ is it correct?
    $endgroup$
    – Vimath
    2 days ago










  • $begingroup$
    Yes, the arrival of buses is independent events.
    $endgroup$
    – IrinaS
    2 days ago










  • $begingroup$
    As a nitpick, the words "probability" and "odds" are not interchangeable. They are related, yes, but they do not mean the same thing. The probability of picking an ace from a well shuffled standard deck is $frac{1}{13}$. The odds however are $1:12$ for, or equivalently $12:1$ against. If you only ever want to talk about probabilities, then only use the word probability and avoid using the word odds. Also, @Vimath conditional probabilities are written with a vertical bar, not a slanted bar. It should be $P(Amid B')$, not $P(A/B')$
    $endgroup$
    – JMoravitz
    2 days ago










  • $begingroup$
    @JMoravitz I missed the syntax there , I'll try not to repeat it.
    $endgroup$
    – Vimath
    2 days ago














6












6








6





$begingroup$


Bus A arrives at a random time between 2pm and 4pm, and Bus B arrives at a random time between 3pm and 5pm. What are the odds that Bus A arrives before Bus B?



My understanding is that since Bus B cannot possibly arrive between 2 and 3, we can only talk about the time between 3 and 4 pm, when there is an equal probability for both buses arriving. But in this case, the probability of Bus A arriving before B is 50%. No? What am I missing here? Or I should look at the entire timeline, 2 pm - 5 pm? But then in this case, it is still 50%. Where is my thinking wrong?










share|cite|improve this question









New contributor




IrinaS is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.







$endgroup$




Bus A arrives at a random time between 2pm and 4pm, and Bus B arrives at a random time between 3pm and 5pm. What are the odds that Bus A arrives before Bus B?



My understanding is that since Bus B cannot possibly arrive between 2 and 3, we can only talk about the time between 3 and 4 pm, when there is an equal probability for both buses arriving. But in this case, the probability of Bus A arriving before B is 50%. No? What am I missing here? Or I should look at the entire timeline, 2 pm - 5 pm? But then in this case, it is still 50%. Where is my thinking wrong?







probability






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edited 2 days ago







IrinaS













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asked 2 days ago









IrinaSIrinaS

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  • 1




    $begingroup$
    Using conditional probability I think it must be. $P(A/B')$ proceeding from here gives us ans $1/22$ is it correct?
    $endgroup$
    – Vimath
    2 days ago










  • $begingroup$
    Yes, the arrival of buses is independent events.
    $endgroup$
    – IrinaS
    2 days ago










  • $begingroup$
    As a nitpick, the words "probability" and "odds" are not interchangeable. They are related, yes, but they do not mean the same thing. The probability of picking an ace from a well shuffled standard deck is $frac{1}{13}$. The odds however are $1:12$ for, or equivalently $12:1$ against. If you only ever want to talk about probabilities, then only use the word probability and avoid using the word odds. Also, @Vimath conditional probabilities are written with a vertical bar, not a slanted bar. It should be $P(Amid B')$, not $P(A/B')$
    $endgroup$
    – JMoravitz
    2 days ago










  • $begingroup$
    @JMoravitz I missed the syntax there , I'll try not to repeat it.
    $endgroup$
    – Vimath
    2 days ago














  • 1




    $begingroup$
    Using conditional probability I think it must be. $P(A/B')$ proceeding from here gives us ans $1/22$ is it correct?
    $endgroup$
    – Vimath
    2 days ago










  • $begingroup$
    Yes, the arrival of buses is independent events.
    $endgroup$
    – IrinaS
    2 days ago










  • $begingroup$
    As a nitpick, the words "probability" and "odds" are not interchangeable. They are related, yes, but they do not mean the same thing. The probability of picking an ace from a well shuffled standard deck is $frac{1}{13}$. The odds however are $1:12$ for, or equivalently $12:1$ against. If you only ever want to talk about probabilities, then only use the word probability and avoid using the word odds. Also, @Vimath conditional probabilities are written with a vertical bar, not a slanted bar. It should be $P(Amid B')$, not $P(A/B')$
    $endgroup$
    – JMoravitz
    2 days ago










  • $begingroup$
    @JMoravitz I missed the syntax there , I'll try not to repeat it.
    $endgroup$
    – Vimath
    2 days ago








1




1




$begingroup$
Using conditional probability I think it must be. $P(A/B')$ proceeding from here gives us ans $1/22$ is it correct?
$endgroup$
– Vimath
2 days ago




$begingroup$
Using conditional probability I think it must be. $P(A/B')$ proceeding from here gives us ans $1/22$ is it correct?
$endgroup$
– Vimath
2 days ago












$begingroup$
Yes, the arrival of buses is independent events.
$endgroup$
– IrinaS
2 days ago




$begingroup$
Yes, the arrival of buses is independent events.
$endgroup$
– IrinaS
2 days ago












$begingroup$
As a nitpick, the words "probability" and "odds" are not interchangeable. They are related, yes, but they do not mean the same thing. The probability of picking an ace from a well shuffled standard deck is $frac{1}{13}$. The odds however are $1:12$ for, or equivalently $12:1$ against. If you only ever want to talk about probabilities, then only use the word probability and avoid using the word odds. Also, @Vimath conditional probabilities are written with a vertical bar, not a slanted bar. It should be $P(Amid B')$, not $P(A/B')$
$endgroup$
– JMoravitz
2 days ago




$begingroup$
As a nitpick, the words "probability" and "odds" are not interchangeable. They are related, yes, but they do not mean the same thing. The probability of picking an ace from a well shuffled standard deck is $frac{1}{13}$. The odds however are $1:12$ for, or equivalently $12:1$ against. If you only ever want to talk about probabilities, then only use the word probability and avoid using the word odds. Also, @Vimath conditional probabilities are written with a vertical bar, not a slanted bar. It should be $P(Amid B')$, not $P(A/B')$
$endgroup$
– JMoravitz
2 days ago












$begingroup$
@JMoravitz I missed the syntax there , I'll try not to repeat it.
$endgroup$
– Vimath
2 days ago




$begingroup$
@JMoravitz I missed the syntax there , I'll try not to repeat it.
$endgroup$
– Vimath
2 days ago










7 Answers
7






active

oldest

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Guide:



1) Draw rectangle $2le xle 4,$ $3le yle 5$.



2) The area of the rectangle is $4$, so pdf is $1/4$.



3) Draw line $y=x$.



4) Find area of the rectangle above the line, which is $7/2$.



5) Finally, the required probability is $7/2cdot 1/4=7/8$.



Here is the graph:



$hspace{2cm}$ enter image description here



Bus $A$ arriving before bus $B$:
$$A(2.5,4.5),B(3.5,4.5),C(2.5,3.5),D(3.4,3.7).$$
Bus $A$ arriving after bus $B$:
$$E(3.7,3.3).$$






share|cite|improve this answer











$endgroup$













  • $begingroup$
    do we need the area above the x=y line, or below the line? I believe - under the line. Can you please clarify?
    $endgroup$
    – IrinaS
    2 days ago










  • $begingroup$
    The graph is a good idea but you need to get rid of the block F/G. Obviously the intersection is 2 hrs by 2 hrs - a square.
    $endgroup$
    – Craig Hicks
    2 days ago










  • $begingroup$
    Thanks, I rolled back to my first answer.
    $endgroup$
    – farruhota
    2 days ago










  • $begingroup$
    But the graph was great! Bring back the graph!
    $endgroup$
    – Craig Hicks
    2 days ago










  • $begingroup$
    my first correct answer was from the phone, then I spoiled by drawing the graph on computer! Just kidding, I will.
    $endgroup$
    – farruhota
    2 days ago



















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First, I’ll assume that the probability distribution of each bus’s arrival time is uniform in its range and independent. I think that’s implicit in the question but you don’t actually say so.



You’re mistaken when you say that if you know Bus A is arriving between $3$ and $4$, then there is an equal probability of either bus arriving first. There is a $50$% probability that Bus B arrives after $4$. The probabilities are equal only if you know that Bus A arrives between $3$ and $4$ and also that Bus B arrives between $3$ and $4$. That parlay occurs only $25$% of the time.



So $75$% of the time, you know that Bus A arrives first, and Bus A still arrives first half of the remaining $25$% of the time. Thus, the probability that Bus A arrives first is $87.5$%.






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    6












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    Let $A_e$ ($A$ early) be the event that bus $A$ arrives before $3$pm.



    Let $B_ell$ ($B$ late) be the event that bus $B$ arrives after $4$pm.



    Let $C$ be the union : $C=A_e cup B_ell$. Hence $$P(C)=P(A_e)+P(B_ell)-P(A_e cap B_ell)=P(A_e)+P(B_ell)-P(A_e)P(B_ell)$$



    Let $X$ be the event of interest ( bus $A$ arrives before bus $B$).



    What we know (don't we?) is that $P(X | C)=1$ and $P(X | overline{C})=0.5$



    Then we can write (total probability) $$P(X) = P(X cap C) + P(X capoverline{C})=P(X | C) P(C) + P(X mid overline{C})P(overline{C})$$



    Can you go on from here ?






    share|cite|improve this answer











    $endgroup$













    • $begingroup$
      Sorry for the question. I am confused with 𝑃(𝑋|𝐶)=1. Here is why - A has already arrived before 4 pm, before 𝐵ℓ happens, right? Yes, B can arrive after 4 pm, but it does not make any difference for the question we are asking which is - the probability of A arriving before B, before 4 pm, because A arrive between 2 and 4.
      $endgroup$
      – IrinaS
      2 days ago








    • 3




      $begingroup$
      @IrinaS - "A has already arrived before $B_ell$ happens" -- that would be $P(B_ell | A_ell)$. And you're right -- if $A_ell$ happens, then A arrives before B no matter what. But leonbloy is talking about the union of $A_ell$ and $B_ell$ -- A arrives before 3, OR B arrives after 4, OR both. In any of these cases, A arrives before B for sure, hence $P(X | A cup B)$ = 1
      $endgroup$
      – cag51
      2 days ago












    • $begingroup$
      @IrinaS What cag51 says above.
      $endgroup$
      – leonbloy
      2 days ago



















    1












    $begingroup$

    Define a new variable
    $Z = A-B = A+ (-B)$.



    Whenever $Z<0 Rightarrow A<B$ (A arrives before B)



    Since A and (-B) are independent, the pdf of Z is the convolution of the pdfs of A and (-B): $f_Z(z) = f_A(a)*f_{-B}(b)$.



    Solving the convolution graphically you get that:
    $$f_Z(z) = cases{ frac{z+3}{4}, -3 leq z < -1
    \ frac{1-z}{4}, -1 leq z < 1}$$



    Convolution between A and -B



    Now compute $P(Z<0)$






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    • $begingroup$
      What I like about this answer - (1) short description to get to computeable equation (2) the graph shows immediately the average, maximum, and minimum times that A arrives before B: 1 hr, 3hr, -1hr respectively, so the result is easy to check.
      $endgroup$
      – Craig Hicks
      2 days ago



















    1












    $begingroup$

    $$int_{s=2}^4 int_{t=3}^5 p(a=s) p(b=t) delta(s<t) $$



    is the 2-d continuous integral equation. $delta(s<t)$ is 1 when $s<t$ and 0 otherwise. $delta(s<t)$ bisects the total area into two parts, the sum of which is 1.



    It is easily visualized and solved with a 2 x 2 hours block of time with a diagonal line $delta(s<t)$ cutting through one corner.



    The graph in farruhota's answer shows it.






    share|cite|improve this answer









    $endgroup$













    • $begingroup$
      I wouldn't say "bisects". In other contexts, bisection implies equal parts.
      $endgroup$
      – David K
      2 days ago










    • $begingroup$
      @DavidK - I agree. "partitions" is a much better word choice.
      $endgroup$
      – Craig Hicks
      2 days ago



















    1












    $begingroup$

    What you missed in your approach to the question is that there is nothing in the problem statement that prevents bus $B$ from arriving after $4$ pm, and in fact (assuming uniform, independent distributions of the arrival times) half the time bus $B$ will arrive after $4$ pm,
    and in that case bus $A$ will have arrived first.



    Likewise, there is nothing that requires bus $A$ to arrive after $3$ pm so that bus $B$ has a chance to arrive first. Bus $A$ can just as likely arrive before $3$ pm.



    Let's take a frequentist approach. Suppose that these two buses run on this random schedule seven days a week, every day of the year. Let's watch them arrive for a few days and see what happens. Here's one possible way this might unfold:




    • On Monday, bus $A$ arrived at $2{:}38$ and bus $B$ arrived at $3{:}02$.
      Although bus $B$ arrived almost as quickly as it possibly can, bus $A$ still arrived first.


    • On Tuesday, bus $A$ arrived at $3{:}05$ and bus $B$ arrived at $3{:}42$.
      Bus $A$ arrived first.


    • On Wednesday, bus $A$ arrived at $2{:}50$ and bus $B$ arrived at $4{:}11$.
      Bus $A$ arrived first.


    • On Thursday, bus $A$ arrived at $3{:}57$ and bus $B$ arrived at $4{:}30$.
      Bus $A$ arrived first even though it was almost as late as it can be.


    • On Friday, bus $A$ arrived at $2{:}05$ and bus $B$ arrived at $4{:}56$.
      Bus $A$ arrived first.


    • On Saturday, bus $A$ arrived at $3{:}19$ and bus $B$ arrived at $3{:}17$.
      Finally we have observed an event in which bus $B$ arrived first!



    In the long run, if we keep track of the relative frequency of days like Monday
    (when bus $A$ arrives before $3$ pm and bus $B$ arrives between $3$ and $4$ pm),
    we'll find that the relative frequency approaches $1/4$ of all the days.
    To put it simply, in the long run $1/4$ of the days will be like Monday.



    Similarly, in the long run $1/4$ of the days will be like Wednesday and Friday, when bus $A$ arrived before $3$ pm and bus $B$ arrived after $4$ pm.
    Another $1/4$ of the days will be like Thursday, when bus $A$ arrived between $3$ and $4$ pm but bus $B$ arrives after $4$ pm.



    That leaves just $1/4$ of the days in the long run when bus $A$ and bus $B$ both arrive between $3$ and $4$ pm. One half of those days ($1/8$ of all days in the long run) will be like Tuesday, when bus $A$ arrived first, and the other half of those days ($1/8$ of all days in the long run) will be like Saturday, when bus $B$ arrived first.



    And that accounts for all possibilities. In one case, which happens $1/8$ of the time, bus $B$ arrives first. In all other cases bus $A$ arrives first.






    share|cite|improve this answer









    $endgroup$













    • $begingroup$
      Agree with your logic. This is a very good and simple explanation. Thank you, @David K.
      $endgroup$
      – IrinaS
      2 days ago



















    0












    $begingroup$

    The joint distribution for arrival times of A and B is
    $$P(A,B)d!Ad!B = frac{1}{4}d!Ad!Bqquad 2<A<4, mathrm{and} ,3<B<5$$



    We need the probability that A is less than B
    $$P(A<B) = int_3^5 d!B int_2^{min(B,4)}d!A = frac{7}{8}$$






    share|cite|improve this answer









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      7 Answers
      7






      active

      oldest

      votes








      7 Answers
      7






      active

      oldest

      votes









      active

      oldest

      votes






      active

      oldest

      votes









      14












      $begingroup$

      Guide:



      1) Draw rectangle $2le xle 4,$ $3le yle 5$.



      2) The area of the rectangle is $4$, so pdf is $1/4$.



      3) Draw line $y=x$.



      4) Find area of the rectangle above the line, which is $7/2$.



      5) Finally, the required probability is $7/2cdot 1/4=7/8$.



      Here is the graph:



      $hspace{2cm}$ enter image description here



      Bus $A$ arriving before bus $B$:
      $$A(2.5,4.5),B(3.5,4.5),C(2.5,3.5),D(3.4,3.7).$$
      Bus $A$ arriving after bus $B$:
      $$E(3.7,3.3).$$






      share|cite|improve this answer











      $endgroup$













      • $begingroup$
        do we need the area above the x=y line, or below the line? I believe - under the line. Can you please clarify?
        $endgroup$
        – IrinaS
        2 days ago










      • $begingroup$
        The graph is a good idea but you need to get rid of the block F/G. Obviously the intersection is 2 hrs by 2 hrs - a square.
        $endgroup$
        – Craig Hicks
        2 days ago










      • $begingroup$
        Thanks, I rolled back to my first answer.
        $endgroup$
        – farruhota
        2 days ago










      • $begingroup$
        But the graph was great! Bring back the graph!
        $endgroup$
        – Craig Hicks
        2 days ago










      • $begingroup$
        my first correct answer was from the phone, then I spoiled by drawing the graph on computer! Just kidding, I will.
        $endgroup$
        – farruhota
        2 days ago
















      14












      $begingroup$

      Guide:



      1) Draw rectangle $2le xle 4,$ $3le yle 5$.



      2) The area of the rectangle is $4$, so pdf is $1/4$.



      3) Draw line $y=x$.



      4) Find area of the rectangle above the line, which is $7/2$.



      5) Finally, the required probability is $7/2cdot 1/4=7/8$.



      Here is the graph:



      $hspace{2cm}$ enter image description here



      Bus $A$ arriving before bus $B$:
      $$A(2.5,4.5),B(3.5,4.5),C(2.5,3.5),D(3.4,3.7).$$
      Bus $A$ arriving after bus $B$:
      $$E(3.7,3.3).$$






      share|cite|improve this answer











      $endgroup$













      • $begingroup$
        do we need the area above the x=y line, or below the line? I believe - under the line. Can you please clarify?
        $endgroup$
        – IrinaS
        2 days ago










      • $begingroup$
        The graph is a good idea but you need to get rid of the block F/G. Obviously the intersection is 2 hrs by 2 hrs - a square.
        $endgroup$
        – Craig Hicks
        2 days ago










      • $begingroup$
        Thanks, I rolled back to my first answer.
        $endgroup$
        – farruhota
        2 days ago










      • $begingroup$
        But the graph was great! Bring back the graph!
        $endgroup$
        – Craig Hicks
        2 days ago










      • $begingroup$
        my first correct answer was from the phone, then I spoiled by drawing the graph on computer! Just kidding, I will.
        $endgroup$
        – farruhota
        2 days ago














      14












      14








      14





      $begingroup$

      Guide:



      1) Draw rectangle $2le xle 4,$ $3le yle 5$.



      2) The area of the rectangle is $4$, so pdf is $1/4$.



      3) Draw line $y=x$.



      4) Find area of the rectangle above the line, which is $7/2$.



      5) Finally, the required probability is $7/2cdot 1/4=7/8$.



      Here is the graph:



      $hspace{2cm}$ enter image description here



      Bus $A$ arriving before bus $B$:
      $$A(2.5,4.5),B(3.5,4.5),C(2.5,3.5),D(3.4,3.7).$$
      Bus $A$ arriving after bus $B$:
      $$E(3.7,3.3).$$






      share|cite|improve this answer











      $endgroup$



      Guide:



      1) Draw rectangle $2le xle 4,$ $3le yle 5$.



      2) The area of the rectangle is $4$, so pdf is $1/4$.



      3) Draw line $y=x$.



      4) Find area of the rectangle above the line, which is $7/2$.



      5) Finally, the required probability is $7/2cdot 1/4=7/8$.



      Here is the graph:



      $hspace{2cm}$ enter image description here



      Bus $A$ arriving before bus $B$:
      $$A(2.5,4.5),B(3.5,4.5),C(2.5,3.5),D(3.4,3.7).$$
      Bus $A$ arriving after bus $B$:
      $$E(3.7,3.3).$$







      share|cite|improve this answer














      share|cite|improve this answer



      share|cite|improve this answer








      edited 2 days ago









      David K

      55.4k344120




      55.4k344120










      answered 2 days ago









      farruhotafarruhota

      21.6k2842




      21.6k2842












      • $begingroup$
        do we need the area above the x=y line, or below the line? I believe - under the line. Can you please clarify?
        $endgroup$
        – IrinaS
        2 days ago










      • $begingroup$
        The graph is a good idea but you need to get rid of the block F/G. Obviously the intersection is 2 hrs by 2 hrs - a square.
        $endgroup$
        – Craig Hicks
        2 days ago










      • $begingroup$
        Thanks, I rolled back to my first answer.
        $endgroup$
        – farruhota
        2 days ago










      • $begingroup$
        But the graph was great! Bring back the graph!
        $endgroup$
        – Craig Hicks
        2 days ago










      • $begingroup$
        my first correct answer was from the phone, then I spoiled by drawing the graph on computer! Just kidding, I will.
        $endgroup$
        – farruhota
        2 days ago


















      • $begingroup$
        do we need the area above the x=y line, or below the line? I believe - under the line. Can you please clarify?
        $endgroup$
        – IrinaS
        2 days ago










      • $begingroup$
        The graph is a good idea but you need to get rid of the block F/G. Obviously the intersection is 2 hrs by 2 hrs - a square.
        $endgroup$
        – Craig Hicks
        2 days ago










      • $begingroup$
        Thanks, I rolled back to my first answer.
        $endgroup$
        – farruhota
        2 days ago










      • $begingroup$
        But the graph was great! Bring back the graph!
        $endgroup$
        – Craig Hicks
        2 days ago










      • $begingroup$
        my first correct answer was from the phone, then I spoiled by drawing the graph on computer! Just kidding, I will.
        $endgroup$
        – farruhota
        2 days ago
















      $begingroup$
      do we need the area above the x=y line, or below the line? I believe - under the line. Can you please clarify?
      $endgroup$
      – IrinaS
      2 days ago




      $begingroup$
      do we need the area above the x=y line, or below the line? I believe - under the line. Can you please clarify?
      $endgroup$
      – IrinaS
      2 days ago












      $begingroup$
      The graph is a good idea but you need to get rid of the block F/G. Obviously the intersection is 2 hrs by 2 hrs - a square.
      $endgroup$
      – Craig Hicks
      2 days ago




      $begingroup$
      The graph is a good idea but you need to get rid of the block F/G. Obviously the intersection is 2 hrs by 2 hrs - a square.
      $endgroup$
      – Craig Hicks
      2 days ago












      $begingroup$
      Thanks, I rolled back to my first answer.
      $endgroup$
      – farruhota
      2 days ago




      $begingroup$
      Thanks, I rolled back to my first answer.
      $endgroup$
      – farruhota
      2 days ago












      $begingroup$
      But the graph was great! Bring back the graph!
      $endgroup$
      – Craig Hicks
      2 days ago




      $begingroup$
      But the graph was great! Bring back the graph!
      $endgroup$
      – Craig Hicks
      2 days ago












      $begingroup$
      my first correct answer was from the phone, then I spoiled by drawing the graph on computer! Just kidding, I will.
      $endgroup$
      – farruhota
      2 days ago




      $begingroup$
      my first correct answer was from the phone, then I spoiled by drawing the graph on computer! Just kidding, I will.
      $endgroup$
      – farruhota
      2 days ago











      12












      $begingroup$

      First, I’ll assume that the probability distribution of each bus’s arrival time is uniform in its range and independent. I think that’s implicit in the question but you don’t actually say so.



      You’re mistaken when you say that if you know Bus A is arriving between $3$ and $4$, then there is an equal probability of either bus arriving first. There is a $50$% probability that Bus B arrives after $4$. The probabilities are equal only if you know that Bus A arrives between $3$ and $4$ and also that Bus B arrives between $3$ and $4$. That parlay occurs only $25$% of the time.



      So $75$% of the time, you know that Bus A arrives first, and Bus A still arrives first half of the remaining $25$% of the time. Thus, the probability that Bus A arrives first is $87.5$%.






      share|cite|improve this answer









      $endgroup$


















        12












        $begingroup$

        First, I’ll assume that the probability distribution of each bus’s arrival time is uniform in its range and independent. I think that’s implicit in the question but you don’t actually say so.



        You’re mistaken when you say that if you know Bus A is arriving between $3$ and $4$, then there is an equal probability of either bus arriving first. There is a $50$% probability that Bus B arrives after $4$. The probabilities are equal only if you know that Bus A arrives between $3$ and $4$ and also that Bus B arrives between $3$ and $4$. That parlay occurs only $25$% of the time.



        So $75$% of the time, you know that Bus A arrives first, and Bus A still arrives first half of the remaining $25$% of the time. Thus, the probability that Bus A arrives first is $87.5$%.






        share|cite|improve this answer









        $endgroup$
















          12












          12








          12





          $begingroup$

          First, I’ll assume that the probability distribution of each bus’s arrival time is uniform in its range and independent. I think that’s implicit in the question but you don’t actually say so.



          You’re mistaken when you say that if you know Bus A is arriving between $3$ and $4$, then there is an equal probability of either bus arriving first. There is a $50$% probability that Bus B arrives after $4$. The probabilities are equal only if you know that Bus A arrives between $3$ and $4$ and also that Bus B arrives between $3$ and $4$. That parlay occurs only $25$% of the time.



          So $75$% of the time, you know that Bus A arrives first, and Bus A still arrives first half of the remaining $25$% of the time. Thus, the probability that Bus A arrives first is $87.5$%.






          share|cite|improve this answer









          $endgroup$



          First, I’ll assume that the probability distribution of each bus’s arrival time is uniform in its range and independent. I think that’s implicit in the question but you don’t actually say so.



          You’re mistaken when you say that if you know Bus A is arriving between $3$ and $4$, then there is an equal probability of either bus arriving first. There is a $50$% probability that Bus B arrives after $4$. The probabilities are equal only if you know that Bus A arrives between $3$ and $4$ and also that Bus B arrives between $3$ and $4$. That parlay occurs only $25$% of the time.



          So $75$% of the time, you know that Bus A arrives first, and Bus A still arrives first half of the remaining $25$% of the time. Thus, the probability that Bus A arrives first is $87.5$%.







          share|cite|improve this answer












          share|cite|improve this answer



          share|cite|improve this answer










          answered 2 days ago









          Robert ShoreRobert Shore

          3,573324




          3,573324























              6












              $begingroup$

              Let $A_e$ ($A$ early) be the event that bus $A$ arrives before $3$pm.



              Let $B_ell$ ($B$ late) be the event that bus $B$ arrives after $4$pm.



              Let $C$ be the union : $C=A_e cup B_ell$. Hence $$P(C)=P(A_e)+P(B_ell)-P(A_e cap B_ell)=P(A_e)+P(B_ell)-P(A_e)P(B_ell)$$



              Let $X$ be the event of interest ( bus $A$ arrives before bus $B$).



              What we know (don't we?) is that $P(X | C)=1$ and $P(X | overline{C})=0.5$



              Then we can write (total probability) $$P(X) = P(X cap C) + P(X capoverline{C})=P(X | C) P(C) + P(X mid overline{C})P(overline{C})$$



              Can you go on from here ?






              share|cite|improve this answer











              $endgroup$













              • $begingroup$
                Sorry for the question. I am confused with 𝑃(𝑋|𝐶)=1. Here is why - A has already arrived before 4 pm, before 𝐵ℓ happens, right? Yes, B can arrive after 4 pm, but it does not make any difference for the question we are asking which is - the probability of A arriving before B, before 4 pm, because A arrive between 2 and 4.
                $endgroup$
                – IrinaS
                2 days ago








              • 3




                $begingroup$
                @IrinaS - "A has already arrived before $B_ell$ happens" -- that would be $P(B_ell | A_ell)$. And you're right -- if $A_ell$ happens, then A arrives before B no matter what. But leonbloy is talking about the union of $A_ell$ and $B_ell$ -- A arrives before 3, OR B arrives after 4, OR both. In any of these cases, A arrives before B for sure, hence $P(X | A cup B)$ = 1
                $endgroup$
                – cag51
                2 days ago












              • $begingroup$
                @IrinaS What cag51 says above.
                $endgroup$
                – leonbloy
                2 days ago
















              6












              $begingroup$

              Let $A_e$ ($A$ early) be the event that bus $A$ arrives before $3$pm.



              Let $B_ell$ ($B$ late) be the event that bus $B$ arrives after $4$pm.



              Let $C$ be the union : $C=A_e cup B_ell$. Hence $$P(C)=P(A_e)+P(B_ell)-P(A_e cap B_ell)=P(A_e)+P(B_ell)-P(A_e)P(B_ell)$$



              Let $X$ be the event of interest ( bus $A$ arrives before bus $B$).



              What we know (don't we?) is that $P(X | C)=1$ and $P(X | overline{C})=0.5$



              Then we can write (total probability) $$P(X) = P(X cap C) + P(X capoverline{C})=P(X | C) P(C) + P(X mid overline{C})P(overline{C})$$



              Can you go on from here ?






              share|cite|improve this answer











              $endgroup$













              • $begingroup$
                Sorry for the question. I am confused with 𝑃(𝑋|𝐶)=1. Here is why - A has already arrived before 4 pm, before 𝐵ℓ happens, right? Yes, B can arrive after 4 pm, but it does not make any difference for the question we are asking which is - the probability of A arriving before B, before 4 pm, because A arrive between 2 and 4.
                $endgroup$
                – IrinaS
                2 days ago








              • 3




                $begingroup$
                @IrinaS - "A has already arrived before $B_ell$ happens" -- that would be $P(B_ell | A_ell)$. And you're right -- if $A_ell$ happens, then A arrives before B no matter what. But leonbloy is talking about the union of $A_ell$ and $B_ell$ -- A arrives before 3, OR B arrives after 4, OR both. In any of these cases, A arrives before B for sure, hence $P(X | A cup B)$ = 1
                $endgroup$
                – cag51
                2 days ago












              • $begingroup$
                @IrinaS What cag51 says above.
                $endgroup$
                – leonbloy
                2 days ago














              6












              6








              6





              $begingroup$

              Let $A_e$ ($A$ early) be the event that bus $A$ arrives before $3$pm.



              Let $B_ell$ ($B$ late) be the event that bus $B$ arrives after $4$pm.



              Let $C$ be the union : $C=A_e cup B_ell$. Hence $$P(C)=P(A_e)+P(B_ell)-P(A_e cap B_ell)=P(A_e)+P(B_ell)-P(A_e)P(B_ell)$$



              Let $X$ be the event of interest ( bus $A$ arrives before bus $B$).



              What we know (don't we?) is that $P(X | C)=1$ and $P(X | overline{C})=0.5$



              Then we can write (total probability) $$P(X) = P(X cap C) + P(X capoverline{C})=P(X | C) P(C) + P(X mid overline{C})P(overline{C})$$



              Can you go on from here ?






              share|cite|improve this answer











              $endgroup$



              Let $A_e$ ($A$ early) be the event that bus $A$ arrives before $3$pm.



              Let $B_ell$ ($B$ late) be the event that bus $B$ arrives after $4$pm.



              Let $C$ be the union : $C=A_e cup B_ell$. Hence $$P(C)=P(A_e)+P(B_ell)-P(A_e cap B_ell)=P(A_e)+P(B_ell)-P(A_e)P(B_ell)$$



              Let $X$ be the event of interest ( bus $A$ arrives before bus $B$).



              What we know (don't we?) is that $P(X | C)=1$ and $P(X | overline{C})=0.5$



              Then we can write (total probability) $$P(X) = P(X cap C) + P(X capoverline{C})=P(X | C) P(C) + P(X mid overline{C})P(overline{C})$$



              Can you go on from here ?







              share|cite|improve this answer














              share|cite|improve this answer



              share|cite|improve this answer








              edited 2 days ago

























              answered 2 days ago









              leonbloyleonbloy

              41.9k647108




              41.9k647108












              • $begingroup$
                Sorry for the question. I am confused with 𝑃(𝑋|𝐶)=1. Here is why - A has already arrived before 4 pm, before 𝐵ℓ happens, right? Yes, B can arrive after 4 pm, but it does not make any difference for the question we are asking which is - the probability of A arriving before B, before 4 pm, because A arrive between 2 and 4.
                $endgroup$
                – IrinaS
                2 days ago








              • 3




                $begingroup$
                @IrinaS - "A has already arrived before $B_ell$ happens" -- that would be $P(B_ell | A_ell)$. And you're right -- if $A_ell$ happens, then A arrives before B no matter what. But leonbloy is talking about the union of $A_ell$ and $B_ell$ -- A arrives before 3, OR B arrives after 4, OR both. In any of these cases, A arrives before B for sure, hence $P(X | A cup B)$ = 1
                $endgroup$
                – cag51
                2 days ago












              • $begingroup$
                @IrinaS What cag51 says above.
                $endgroup$
                – leonbloy
                2 days ago


















              • $begingroup$
                Sorry for the question. I am confused with 𝑃(𝑋|𝐶)=1. Here is why - A has already arrived before 4 pm, before 𝐵ℓ happens, right? Yes, B can arrive after 4 pm, but it does not make any difference for the question we are asking which is - the probability of A arriving before B, before 4 pm, because A arrive between 2 and 4.
                $endgroup$
                – IrinaS
                2 days ago








              • 3




                $begingroup$
                @IrinaS - "A has already arrived before $B_ell$ happens" -- that would be $P(B_ell | A_ell)$. And you're right -- if $A_ell$ happens, then A arrives before B no matter what. But leonbloy is talking about the union of $A_ell$ and $B_ell$ -- A arrives before 3, OR B arrives after 4, OR both. In any of these cases, A arrives before B for sure, hence $P(X | A cup B)$ = 1
                $endgroup$
                – cag51
                2 days ago












              • $begingroup$
                @IrinaS What cag51 says above.
                $endgroup$
                – leonbloy
                2 days ago
















              $begingroup$
              Sorry for the question. I am confused with 𝑃(𝑋|𝐶)=1. Here is why - A has already arrived before 4 pm, before 𝐵ℓ happens, right? Yes, B can arrive after 4 pm, but it does not make any difference for the question we are asking which is - the probability of A arriving before B, before 4 pm, because A arrive between 2 and 4.
              $endgroup$
              – IrinaS
              2 days ago






              $begingroup$
              Sorry for the question. I am confused with 𝑃(𝑋|𝐶)=1. Here is why - A has already arrived before 4 pm, before 𝐵ℓ happens, right? Yes, B can arrive after 4 pm, but it does not make any difference for the question we are asking which is - the probability of A arriving before B, before 4 pm, because A arrive between 2 and 4.
              $endgroup$
              – IrinaS
              2 days ago






              3




              3




              $begingroup$
              @IrinaS - "A has already arrived before $B_ell$ happens" -- that would be $P(B_ell | A_ell)$. And you're right -- if $A_ell$ happens, then A arrives before B no matter what. But leonbloy is talking about the union of $A_ell$ and $B_ell$ -- A arrives before 3, OR B arrives after 4, OR both. In any of these cases, A arrives before B for sure, hence $P(X | A cup B)$ = 1
              $endgroup$
              – cag51
              2 days ago






              $begingroup$
              @IrinaS - "A has already arrived before $B_ell$ happens" -- that would be $P(B_ell | A_ell)$. And you're right -- if $A_ell$ happens, then A arrives before B no matter what. But leonbloy is talking about the union of $A_ell$ and $B_ell$ -- A arrives before 3, OR B arrives after 4, OR both. In any of these cases, A arrives before B for sure, hence $P(X | A cup B)$ = 1
              $endgroup$
              – cag51
              2 days ago














              $begingroup$
              @IrinaS What cag51 says above.
              $endgroup$
              – leonbloy
              2 days ago




              $begingroup$
              @IrinaS What cag51 says above.
              $endgroup$
              – leonbloy
              2 days ago











              1












              $begingroup$

              Define a new variable
              $Z = A-B = A+ (-B)$.



              Whenever $Z<0 Rightarrow A<B$ (A arrives before B)



              Since A and (-B) are independent, the pdf of Z is the convolution of the pdfs of A and (-B): $f_Z(z) = f_A(a)*f_{-B}(b)$.



              Solving the convolution graphically you get that:
              $$f_Z(z) = cases{ frac{z+3}{4}, -3 leq z < -1
              \ frac{1-z}{4}, -1 leq z < 1}$$



              Convolution between A and -B



              Now compute $P(Z<0)$






              share|cite|improve this answer








              New contributor




              maz is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
              Check out our Code of Conduct.






              $endgroup$













              • $begingroup$
                What I like about this answer - (1) short description to get to computeable equation (2) the graph shows immediately the average, maximum, and minimum times that A arrives before B: 1 hr, 3hr, -1hr respectively, so the result is easy to check.
                $endgroup$
                – Craig Hicks
                2 days ago
















              1












              $begingroup$

              Define a new variable
              $Z = A-B = A+ (-B)$.



              Whenever $Z<0 Rightarrow A<B$ (A arrives before B)



              Since A and (-B) are independent, the pdf of Z is the convolution of the pdfs of A and (-B): $f_Z(z) = f_A(a)*f_{-B}(b)$.



              Solving the convolution graphically you get that:
              $$f_Z(z) = cases{ frac{z+3}{4}, -3 leq z < -1
              \ frac{1-z}{4}, -1 leq z < 1}$$



              Convolution between A and -B



              Now compute $P(Z<0)$






              share|cite|improve this answer








              New contributor




              maz is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
              Check out our Code of Conduct.






              $endgroup$













              • $begingroup$
                What I like about this answer - (1) short description to get to computeable equation (2) the graph shows immediately the average, maximum, and minimum times that A arrives before B: 1 hr, 3hr, -1hr respectively, so the result is easy to check.
                $endgroup$
                – Craig Hicks
                2 days ago














              1












              1








              1





              $begingroup$

              Define a new variable
              $Z = A-B = A+ (-B)$.



              Whenever $Z<0 Rightarrow A<B$ (A arrives before B)



              Since A and (-B) are independent, the pdf of Z is the convolution of the pdfs of A and (-B): $f_Z(z) = f_A(a)*f_{-B}(b)$.



              Solving the convolution graphically you get that:
              $$f_Z(z) = cases{ frac{z+3}{4}, -3 leq z < -1
              \ frac{1-z}{4}, -1 leq z < 1}$$



              Convolution between A and -B



              Now compute $P(Z<0)$






              share|cite|improve this answer








              New contributor




              maz is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
              Check out our Code of Conduct.






              $endgroup$



              Define a new variable
              $Z = A-B = A+ (-B)$.



              Whenever $Z<0 Rightarrow A<B$ (A arrives before B)



              Since A and (-B) are independent, the pdf of Z is the convolution of the pdfs of A and (-B): $f_Z(z) = f_A(a)*f_{-B}(b)$.



              Solving the convolution graphically you get that:
              $$f_Z(z) = cases{ frac{z+3}{4}, -3 leq z < -1
              \ frac{1-z}{4}, -1 leq z < 1}$$



              Convolution between A and -B



              Now compute $P(Z<0)$







              share|cite|improve this answer








              New contributor




              maz is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
              Check out our Code of Conduct.









              share|cite|improve this answer



              share|cite|improve this answer






              New contributor




              maz is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
              Check out our Code of Conduct.









              answered 2 days ago









              mazmaz

              1112




              1112




              New contributor




              maz is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
              Check out our Code of Conduct.





              New contributor





              maz is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
              Check out our Code of Conduct.






              maz is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
              Check out our Code of Conduct.












              • $begingroup$
                What I like about this answer - (1) short description to get to computeable equation (2) the graph shows immediately the average, maximum, and minimum times that A arrives before B: 1 hr, 3hr, -1hr respectively, so the result is easy to check.
                $endgroup$
                – Craig Hicks
                2 days ago


















              • $begingroup$
                What I like about this answer - (1) short description to get to computeable equation (2) the graph shows immediately the average, maximum, and minimum times that A arrives before B: 1 hr, 3hr, -1hr respectively, so the result is easy to check.
                $endgroup$
                – Craig Hicks
                2 days ago
















              $begingroup$
              What I like about this answer - (1) short description to get to computeable equation (2) the graph shows immediately the average, maximum, and minimum times that A arrives before B: 1 hr, 3hr, -1hr respectively, so the result is easy to check.
              $endgroup$
              – Craig Hicks
              2 days ago




              $begingroup$
              What I like about this answer - (1) short description to get to computeable equation (2) the graph shows immediately the average, maximum, and minimum times that A arrives before B: 1 hr, 3hr, -1hr respectively, so the result is easy to check.
              $endgroup$
              – Craig Hicks
              2 days ago











              1












              $begingroup$

              $$int_{s=2}^4 int_{t=3}^5 p(a=s) p(b=t) delta(s<t) $$



              is the 2-d continuous integral equation. $delta(s<t)$ is 1 when $s<t$ and 0 otherwise. $delta(s<t)$ bisects the total area into two parts, the sum of which is 1.



              It is easily visualized and solved with a 2 x 2 hours block of time with a diagonal line $delta(s<t)$ cutting through one corner.



              The graph in farruhota's answer shows it.






              share|cite|improve this answer









              $endgroup$













              • $begingroup$
                I wouldn't say "bisects". In other contexts, bisection implies equal parts.
                $endgroup$
                – David K
                2 days ago










              • $begingroup$
                @DavidK - I agree. "partitions" is a much better word choice.
                $endgroup$
                – Craig Hicks
                2 days ago
















              1












              $begingroup$

              $$int_{s=2}^4 int_{t=3}^5 p(a=s) p(b=t) delta(s<t) $$



              is the 2-d continuous integral equation. $delta(s<t)$ is 1 when $s<t$ and 0 otherwise. $delta(s<t)$ bisects the total area into two parts, the sum of which is 1.



              It is easily visualized and solved with a 2 x 2 hours block of time with a diagonal line $delta(s<t)$ cutting through one corner.



              The graph in farruhota's answer shows it.






              share|cite|improve this answer









              $endgroup$













              • $begingroup$
                I wouldn't say "bisects". In other contexts, bisection implies equal parts.
                $endgroup$
                – David K
                2 days ago










              • $begingroup$
                @DavidK - I agree. "partitions" is a much better word choice.
                $endgroup$
                – Craig Hicks
                2 days ago














              1












              1








              1





              $begingroup$

              $$int_{s=2}^4 int_{t=3}^5 p(a=s) p(b=t) delta(s<t) $$



              is the 2-d continuous integral equation. $delta(s<t)$ is 1 when $s<t$ and 0 otherwise. $delta(s<t)$ bisects the total area into two parts, the sum of which is 1.



              It is easily visualized and solved with a 2 x 2 hours block of time with a diagonal line $delta(s<t)$ cutting through one corner.



              The graph in farruhota's answer shows it.






              share|cite|improve this answer









              $endgroup$



              $$int_{s=2}^4 int_{t=3}^5 p(a=s) p(b=t) delta(s<t) $$



              is the 2-d continuous integral equation. $delta(s<t)$ is 1 when $s<t$ and 0 otherwise. $delta(s<t)$ bisects the total area into two parts, the sum of which is 1.



              It is easily visualized and solved with a 2 x 2 hours block of time with a diagonal line $delta(s<t)$ cutting through one corner.



              The graph in farruhota's answer shows it.







              share|cite|improve this answer












              share|cite|improve this answer



              share|cite|improve this answer










              answered 2 days ago









              Craig HicksCraig Hicks

              1687




              1687












              • $begingroup$
                I wouldn't say "bisects". In other contexts, bisection implies equal parts.
                $endgroup$
                – David K
                2 days ago










              • $begingroup$
                @DavidK - I agree. "partitions" is a much better word choice.
                $endgroup$
                – Craig Hicks
                2 days ago


















              • $begingroup$
                I wouldn't say "bisects". In other contexts, bisection implies equal parts.
                $endgroup$
                – David K
                2 days ago










              • $begingroup$
                @DavidK - I agree. "partitions" is a much better word choice.
                $endgroup$
                – Craig Hicks
                2 days ago
















              $begingroup$
              I wouldn't say "bisects". In other contexts, bisection implies equal parts.
              $endgroup$
              – David K
              2 days ago




              $begingroup$
              I wouldn't say "bisects". In other contexts, bisection implies equal parts.
              $endgroup$
              – David K
              2 days ago












              $begingroup$
              @DavidK - I agree. "partitions" is a much better word choice.
              $endgroup$
              – Craig Hicks
              2 days ago




              $begingroup$
              @DavidK - I agree. "partitions" is a much better word choice.
              $endgroup$
              – Craig Hicks
              2 days ago











              1












              $begingroup$

              What you missed in your approach to the question is that there is nothing in the problem statement that prevents bus $B$ from arriving after $4$ pm, and in fact (assuming uniform, independent distributions of the arrival times) half the time bus $B$ will arrive after $4$ pm,
              and in that case bus $A$ will have arrived first.



              Likewise, there is nothing that requires bus $A$ to arrive after $3$ pm so that bus $B$ has a chance to arrive first. Bus $A$ can just as likely arrive before $3$ pm.



              Let's take a frequentist approach. Suppose that these two buses run on this random schedule seven days a week, every day of the year. Let's watch them arrive for a few days and see what happens. Here's one possible way this might unfold:




              • On Monday, bus $A$ arrived at $2{:}38$ and bus $B$ arrived at $3{:}02$.
                Although bus $B$ arrived almost as quickly as it possibly can, bus $A$ still arrived first.


              • On Tuesday, bus $A$ arrived at $3{:}05$ and bus $B$ arrived at $3{:}42$.
                Bus $A$ arrived first.


              • On Wednesday, bus $A$ arrived at $2{:}50$ and bus $B$ arrived at $4{:}11$.
                Bus $A$ arrived first.


              • On Thursday, bus $A$ arrived at $3{:}57$ and bus $B$ arrived at $4{:}30$.
                Bus $A$ arrived first even though it was almost as late as it can be.


              • On Friday, bus $A$ arrived at $2{:}05$ and bus $B$ arrived at $4{:}56$.
                Bus $A$ arrived first.


              • On Saturday, bus $A$ arrived at $3{:}19$ and bus $B$ arrived at $3{:}17$.
                Finally we have observed an event in which bus $B$ arrived first!



              In the long run, if we keep track of the relative frequency of days like Monday
              (when bus $A$ arrives before $3$ pm and bus $B$ arrives between $3$ and $4$ pm),
              we'll find that the relative frequency approaches $1/4$ of all the days.
              To put it simply, in the long run $1/4$ of the days will be like Monday.



              Similarly, in the long run $1/4$ of the days will be like Wednesday and Friday, when bus $A$ arrived before $3$ pm and bus $B$ arrived after $4$ pm.
              Another $1/4$ of the days will be like Thursday, when bus $A$ arrived between $3$ and $4$ pm but bus $B$ arrives after $4$ pm.



              That leaves just $1/4$ of the days in the long run when bus $A$ and bus $B$ both arrive between $3$ and $4$ pm. One half of those days ($1/8$ of all days in the long run) will be like Tuesday, when bus $A$ arrived first, and the other half of those days ($1/8$ of all days in the long run) will be like Saturday, when bus $B$ arrived first.



              And that accounts for all possibilities. In one case, which happens $1/8$ of the time, bus $B$ arrives first. In all other cases bus $A$ arrives first.






              share|cite|improve this answer









              $endgroup$













              • $begingroup$
                Agree with your logic. This is a very good and simple explanation. Thank you, @David K.
                $endgroup$
                – IrinaS
                2 days ago
















              1












              $begingroup$

              What you missed in your approach to the question is that there is nothing in the problem statement that prevents bus $B$ from arriving after $4$ pm, and in fact (assuming uniform, independent distributions of the arrival times) half the time bus $B$ will arrive after $4$ pm,
              and in that case bus $A$ will have arrived first.



              Likewise, there is nothing that requires bus $A$ to arrive after $3$ pm so that bus $B$ has a chance to arrive first. Bus $A$ can just as likely arrive before $3$ pm.



              Let's take a frequentist approach. Suppose that these two buses run on this random schedule seven days a week, every day of the year. Let's watch them arrive for a few days and see what happens. Here's one possible way this might unfold:




              • On Monday, bus $A$ arrived at $2{:}38$ and bus $B$ arrived at $3{:}02$.
                Although bus $B$ arrived almost as quickly as it possibly can, bus $A$ still arrived first.


              • On Tuesday, bus $A$ arrived at $3{:}05$ and bus $B$ arrived at $3{:}42$.
                Bus $A$ arrived first.


              • On Wednesday, bus $A$ arrived at $2{:}50$ and bus $B$ arrived at $4{:}11$.
                Bus $A$ arrived first.


              • On Thursday, bus $A$ arrived at $3{:}57$ and bus $B$ arrived at $4{:}30$.
                Bus $A$ arrived first even though it was almost as late as it can be.


              • On Friday, bus $A$ arrived at $2{:}05$ and bus $B$ arrived at $4{:}56$.
                Bus $A$ arrived first.


              • On Saturday, bus $A$ arrived at $3{:}19$ and bus $B$ arrived at $3{:}17$.
                Finally we have observed an event in which bus $B$ arrived first!



              In the long run, if we keep track of the relative frequency of days like Monday
              (when bus $A$ arrives before $3$ pm and bus $B$ arrives between $3$ and $4$ pm),
              we'll find that the relative frequency approaches $1/4$ of all the days.
              To put it simply, in the long run $1/4$ of the days will be like Monday.



              Similarly, in the long run $1/4$ of the days will be like Wednesday and Friday, when bus $A$ arrived before $3$ pm and bus $B$ arrived after $4$ pm.
              Another $1/4$ of the days will be like Thursday, when bus $A$ arrived between $3$ and $4$ pm but bus $B$ arrives after $4$ pm.



              That leaves just $1/4$ of the days in the long run when bus $A$ and bus $B$ both arrive between $3$ and $4$ pm. One half of those days ($1/8$ of all days in the long run) will be like Tuesday, when bus $A$ arrived first, and the other half of those days ($1/8$ of all days in the long run) will be like Saturday, when bus $B$ arrived first.



              And that accounts for all possibilities. In one case, which happens $1/8$ of the time, bus $B$ arrives first. In all other cases bus $A$ arrives first.






              share|cite|improve this answer









              $endgroup$













              • $begingroup$
                Agree with your logic. This is a very good and simple explanation. Thank you, @David K.
                $endgroup$
                – IrinaS
                2 days ago














              1












              1








              1





              $begingroup$

              What you missed in your approach to the question is that there is nothing in the problem statement that prevents bus $B$ from arriving after $4$ pm, and in fact (assuming uniform, independent distributions of the arrival times) half the time bus $B$ will arrive after $4$ pm,
              and in that case bus $A$ will have arrived first.



              Likewise, there is nothing that requires bus $A$ to arrive after $3$ pm so that bus $B$ has a chance to arrive first. Bus $A$ can just as likely arrive before $3$ pm.



              Let's take a frequentist approach. Suppose that these two buses run on this random schedule seven days a week, every day of the year. Let's watch them arrive for a few days and see what happens. Here's one possible way this might unfold:




              • On Monday, bus $A$ arrived at $2{:}38$ and bus $B$ arrived at $3{:}02$.
                Although bus $B$ arrived almost as quickly as it possibly can, bus $A$ still arrived first.


              • On Tuesday, bus $A$ arrived at $3{:}05$ and bus $B$ arrived at $3{:}42$.
                Bus $A$ arrived first.


              • On Wednesday, bus $A$ arrived at $2{:}50$ and bus $B$ arrived at $4{:}11$.
                Bus $A$ arrived first.


              • On Thursday, bus $A$ arrived at $3{:}57$ and bus $B$ arrived at $4{:}30$.
                Bus $A$ arrived first even though it was almost as late as it can be.


              • On Friday, bus $A$ arrived at $2{:}05$ and bus $B$ arrived at $4{:}56$.
                Bus $A$ arrived first.


              • On Saturday, bus $A$ arrived at $3{:}19$ and bus $B$ arrived at $3{:}17$.
                Finally we have observed an event in which bus $B$ arrived first!



              In the long run, if we keep track of the relative frequency of days like Monday
              (when bus $A$ arrives before $3$ pm and bus $B$ arrives between $3$ and $4$ pm),
              we'll find that the relative frequency approaches $1/4$ of all the days.
              To put it simply, in the long run $1/4$ of the days will be like Monday.



              Similarly, in the long run $1/4$ of the days will be like Wednesday and Friday, when bus $A$ arrived before $3$ pm and bus $B$ arrived after $4$ pm.
              Another $1/4$ of the days will be like Thursday, when bus $A$ arrived between $3$ and $4$ pm but bus $B$ arrives after $4$ pm.



              That leaves just $1/4$ of the days in the long run when bus $A$ and bus $B$ both arrive between $3$ and $4$ pm. One half of those days ($1/8$ of all days in the long run) will be like Tuesday, when bus $A$ arrived first, and the other half of those days ($1/8$ of all days in the long run) will be like Saturday, when bus $B$ arrived first.



              And that accounts for all possibilities. In one case, which happens $1/8$ of the time, bus $B$ arrives first. In all other cases bus $A$ arrives first.






              share|cite|improve this answer









              $endgroup$



              What you missed in your approach to the question is that there is nothing in the problem statement that prevents bus $B$ from arriving after $4$ pm, and in fact (assuming uniform, independent distributions of the arrival times) half the time bus $B$ will arrive after $4$ pm,
              and in that case bus $A$ will have arrived first.



              Likewise, there is nothing that requires bus $A$ to arrive after $3$ pm so that bus $B$ has a chance to arrive first. Bus $A$ can just as likely arrive before $3$ pm.



              Let's take a frequentist approach. Suppose that these two buses run on this random schedule seven days a week, every day of the year. Let's watch them arrive for a few days and see what happens. Here's one possible way this might unfold:




              • On Monday, bus $A$ arrived at $2{:}38$ and bus $B$ arrived at $3{:}02$.
                Although bus $B$ arrived almost as quickly as it possibly can, bus $A$ still arrived first.


              • On Tuesday, bus $A$ arrived at $3{:}05$ and bus $B$ arrived at $3{:}42$.
                Bus $A$ arrived first.


              • On Wednesday, bus $A$ arrived at $2{:}50$ and bus $B$ arrived at $4{:}11$.
                Bus $A$ arrived first.


              • On Thursday, bus $A$ arrived at $3{:}57$ and bus $B$ arrived at $4{:}30$.
                Bus $A$ arrived first even though it was almost as late as it can be.


              • On Friday, bus $A$ arrived at $2{:}05$ and bus $B$ arrived at $4{:}56$.
                Bus $A$ arrived first.


              • On Saturday, bus $A$ arrived at $3{:}19$ and bus $B$ arrived at $3{:}17$.
                Finally we have observed an event in which bus $B$ arrived first!



              In the long run, if we keep track of the relative frequency of days like Monday
              (when bus $A$ arrives before $3$ pm and bus $B$ arrives between $3$ and $4$ pm),
              we'll find that the relative frequency approaches $1/4$ of all the days.
              To put it simply, in the long run $1/4$ of the days will be like Monday.



              Similarly, in the long run $1/4$ of the days will be like Wednesday and Friday, when bus $A$ arrived before $3$ pm and bus $B$ arrived after $4$ pm.
              Another $1/4$ of the days will be like Thursday, when bus $A$ arrived between $3$ and $4$ pm but bus $B$ arrives after $4$ pm.



              That leaves just $1/4$ of the days in the long run when bus $A$ and bus $B$ both arrive between $3$ and $4$ pm. One half of those days ($1/8$ of all days in the long run) will be like Tuesday, when bus $A$ arrived first, and the other half of those days ($1/8$ of all days in the long run) will be like Saturday, when bus $B$ arrived first.



              And that accounts for all possibilities. In one case, which happens $1/8$ of the time, bus $B$ arrives first. In all other cases bus $A$ arrives first.







              share|cite|improve this answer












              share|cite|improve this answer



              share|cite|improve this answer










              answered 2 days ago









              David KDavid K

              55.4k344120




              55.4k344120












              • $begingroup$
                Agree with your logic. This is a very good and simple explanation. Thank you, @David K.
                $endgroup$
                – IrinaS
                2 days ago


















              • $begingroup$
                Agree with your logic. This is a very good and simple explanation. Thank you, @David K.
                $endgroup$
                – IrinaS
                2 days ago
















              $begingroup$
              Agree with your logic. This is a very good and simple explanation. Thank you, @David K.
              $endgroup$
              – IrinaS
              2 days ago




              $begingroup$
              Agree with your logic. This is a very good and simple explanation. Thank you, @David K.
              $endgroup$
              – IrinaS
              2 days ago











              0












              $begingroup$

              The joint distribution for arrival times of A and B is
              $$P(A,B)d!Ad!B = frac{1}{4}d!Ad!Bqquad 2<A<4, mathrm{and} ,3<B<5$$



              We need the probability that A is less than B
              $$P(A<B) = int_3^5 d!B int_2^{min(B,4)}d!A = frac{7}{8}$$






              share|cite|improve this answer









              $endgroup$


















                0












                $begingroup$

                The joint distribution for arrival times of A and B is
                $$P(A,B)d!Ad!B = frac{1}{4}d!Ad!Bqquad 2<A<4, mathrm{and} ,3<B<5$$



                We need the probability that A is less than B
                $$P(A<B) = int_3^5 d!B int_2^{min(B,4)}d!A = frac{7}{8}$$






                share|cite|improve this answer









                $endgroup$
















                  0












                  0








                  0





                  $begingroup$

                  The joint distribution for arrival times of A and B is
                  $$P(A,B)d!Ad!B = frac{1}{4}d!Ad!Bqquad 2<A<4, mathrm{and} ,3<B<5$$



                  We need the probability that A is less than B
                  $$P(A<B) = int_3^5 d!B int_2^{min(B,4)}d!A = frac{7}{8}$$






                  share|cite|improve this answer









                  $endgroup$



                  The joint distribution for arrival times of A and B is
                  $$P(A,B)d!Ad!B = frac{1}{4}d!Ad!Bqquad 2<A<4, mathrm{and} ,3<B<5$$



                  We need the probability that A is less than B
                  $$P(A<B) = int_3^5 d!B int_2^{min(B,4)}d!A = frac{7}{8}$$







                  share|cite|improve this answer












                  share|cite|improve this answer



                  share|cite|improve this answer










                  answered 2 days ago









                  user3856370user3856370

                  1514




                  1514






















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