The probability of Bus A arriving before Bus B
$begingroup$
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?
probability
New contributor
$endgroup$
add a comment |
$begingroup$
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?
probability
New contributor
$endgroup$
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
add a comment |
$begingroup$
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?
probability
New contributor
$endgroup$
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
probability
New contributor
New contributor
edited 2 days ago
IrinaS
New contributor
asked 2 days ago
IrinaSIrinaS
312
312
New contributor
New contributor
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
add a comment |
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
add a comment |
7 Answers
7
active
oldest
votes
$begingroup$
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:
$hspace{2cm}$
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 $B$:
$$E(3.7,3.3).$$
$endgroup$
$begingroup$
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
|
show 6 more comments
$begingroup$
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 $87.5$%.
$endgroup$
add a comment |
$begingroup$
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 ?
$endgroup$
$begingroup$
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
add a comment |
$begingroup$
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:
$$f_Z(z) = cases{ frac{z+3}{4}, -3 leq z < -1
\ frac{1-z}{4}, -1 leq z < 1}$$
Now compute $P(Z<0)$
New contributor
$endgroup$
$begingroup$
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
add a comment |
$begingroup$
$$int_{s=2}^4 int_{t=3}^5 p(a=s) p(b=t) delta(s<t) $$
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.
$endgroup$
$begingroup$
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
add a comment |
$begingroup$
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.
$endgroup$
$begingroup$
Agree with your logic. This is a very good and simple explanation. Thank you, @David K.
$endgroup$
– IrinaS
2 days ago
add a comment |
$begingroup$
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
$$P(A<B) = int_3^5 d!B int_2^{min(B,4)}d!A = frac{7}{8}$$
$endgroup$
add a comment |
Your Answer
StackExchange.ifUsing("editor", function () {
return StackExchange.using("mathjaxEditing", function () {
StackExchange.MarkdownEditor.creationCallbacks.add(function (editor, postfix) {
StackExchange.mathjaxEditing.prepareWmdForMathJax(editor, postfix, [["$", "$"], ["\\(","\\)"]]);
});
});
}, "mathjax-editing");
StackExchange.ready(function() {
var channelOptions = {
tags: "".split(" "),
id: "69"
};
initTagRenderer("".split(" "), "".split(" "), channelOptions);
StackExchange.using("externalEditor", function() {
// Have to fire editor after snippets, if snippets enabled
if (StackExchange.settings.snippets.snippetsEnabled) {
StackExchange.using("snippets", function() {
createEditor();
});
}
else {
createEditor();
}
});
function createEditor() {
StackExchange.prepareEditor({
heartbeatType: 'answer',
autoActivateHeartbeat: false,
convertImagesToLinks: true,
noModals: true,
showLowRepImageUploadWarning: true,
reputationToPostImages: 10,
bindNavPrevention: true,
postfix: "",
imageUploader: {
brandingHtml: "Powered by u003ca class="icon-imgur-white" href="https://imgur.com/"u003eu003c/au003e",
contentPolicyHtml: "User contributions licensed under u003ca href="https://creativecommons.org/licenses/by-sa/3.0/"u003ecc by-sa 3.0 with attribution requiredu003c/au003e u003ca href="https://stackoverflow.com/legal/content-policy"u003e(content policy)u003c/au003e",
allowUrls: true
},
noCode: true, onDemand: true,
discardSelector: ".discard-answer"
,immediatelyShowMarkdownHelp:true
});
}
});
IrinaS is a new contributor. Be nice, and check out our Code of Conduct.
Sign up or log in
StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
StackExchange.ready(
function () {
StackExchange.openid.initPostLogin('.new-post-login', 'https%3a%2f%2fmath.stackexchange.com%2fquestions%2f3158927%2fthe-probability-of-bus-a-arriving-before-bus-b%23new-answer', 'question_page');
}
);
Post as a guest
Required, but never shown
7 Answers
7
active
oldest
votes
7 Answers
7
active
oldest
votes
active
oldest
votes
active
oldest
votes
$begingroup$
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:
$hspace{2cm}$
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 $B$:
$$E(3.7,3.3).$$
$endgroup$
$begingroup$
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
|
show 6 more comments
$begingroup$
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:
$hspace{2cm}$
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 $B$:
$$E(3.7,3.3).$$
$endgroup$
$begingroup$
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
|
show 6 more comments
$begingroup$
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:
$hspace{2cm}$
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 $B$:
$$E(3.7,3.3).$$
$endgroup$
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:
$hspace{2cm}$
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 $B$:
$$E(3.7,3.3).$$
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
|
show 6 more comments
$begingroup$
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
|
show 6 more comments
$begingroup$
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 $87.5$%.
$endgroup$
add a comment |
$begingroup$
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 $87.5$%.
$endgroup$
add a comment |
$begingroup$
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 $87.5$%.
$endgroup$
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 $87.5$%.
answered 2 days ago
Robert ShoreRobert Shore
3,573324
3,573324
add a comment |
add a comment |
$begingroup$
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 ?
$endgroup$
$begingroup$
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
add a comment |
$begingroup$
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 ?
$endgroup$
$begingroup$
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
add a comment |
$begingroup$
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 ?
$endgroup$
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 ?
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
add a comment |
$begingroup$
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
add a comment |
$begingroup$
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:
$$f_Z(z) = cases{ frac{z+3}{4}, -3 leq z < -1
\ frac{1-z}{4}, -1 leq z < 1}$$
Now compute $P(Z<0)$
New contributor
$endgroup$
$begingroup$
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
add a comment |
$begingroup$
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:
$$f_Z(z) = cases{ frac{z+3}{4}, -3 leq z < -1
\ frac{1-z}{4}, -1 leq z < 1}$$
Now compute $P(Z<0)$
New contributor
$endgroup$
$begingroup$
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
add a comment |
$begingroup$
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:
$$f_Z(z) = cases{ frac{z+3}{4}, -3 leq z < -1
\ frac{1-z}{4}, -1 leq z < 1}$$
Now compute $P(Z<0)$
New contributor
$endgroup$
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:
$$f_Z(z) = cases{ frac{z+3}{4}, -3 leq z < -1
\ frac{1-z}{4}, -1 leq z < 1}$$
Now compute $P(Z<0)$
New contributor
New contributor
answered 2 days ago
mazmaz
1112
1112
New contributor
New contributor
$begingroup$
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
add a comment |
$begingroup$
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
add a comment |
$begingroup$
$$int_{s=2}^4 int_{t=3}^5 p(a=s) p(b=t) delta(s<t) $$
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.
$endgroup$
$begingroup$
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
add a comment |
$begingroup$
$$int_{s=2}^4 int_{t=3}^5 p(a=s) p(b=t) delta(s<t) $$
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.
$endgroup$
$begingroup$
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
add a comment |
$begingroup$
$$int_{s=2}^4 int_{t=3}^5 p(a=s) p(b=t) delta(s<t) $$
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.
$endgroup$
$$int_{s=2}^4 int_{t=3}^5 p(a=s) p(b=t) delta(s<t) $$
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.
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
add a comment |
$begingroup$
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
add a comment |
$begingroup$
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.
$endgroup$
$begingroup$
Agree with your logic. This is a very good and simple explanation. Thank you, @David K.
$endgroup$
– IrinaS
2 days ago
add a comment |
$begingroup$
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.
$endgroup$
$begingroup$
Agree with your logic. This is a very good and simple explanation. Thank you, @David K.
$endgroup$
– IrinaS
2 days ago
add a comment |
$begingroup$
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.
$endgroup$
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.
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
add a comment |
$begingroup$
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
add a comment |
$begingroup$
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
$$P(A<B) = int_3^5 d!B int_2^{min(B,4)}d!A = frac{7}{8}$$
$endgroup$
add a comment |
$begingroup$
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
$$P(A<B) = int_3^5 d!B int_2^{min(B,4)}d!A = frac{7}{8}$$
$endgroup$
add a comment |
$begingroup$
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
$$P(A<B) = int_3^5 d!B int_2^{min(B,4)}d!A = frac{7}{8}$$
$endgroup$
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
$$P(A<B) = int_3^5 d!B int_2^{min(B,4)}d!A = frac{7}{8}$$
answered 2 days ago
user3856370user3856370
1514
1514
add a comment |
add a comment |
IrinaS is a new contributor. Be nice, and check out our Code of Conduct.
IrinaS is a new contributor. Be nice, and check out our Code of Conduct.
IrinaS is a new contributor. Be nice, and check out our Code of Conduct.
IrinaS is a new contributor. Be nice, and check out our Code of Conduct.
Thanks for contributing an answer to Mathematics Stack Exchange!
- Please be sure to answer the question. Provide details and share your research!
But avoid …
- Asking for help, clarification, or responding to other answers.
- Making statements based on opinion; back them up with references or personal experience.
Use MathJax to format equations. MathJax reference.
To learn more, see our tips on writing great answers.
Sign up or log in
StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
StackExchange.ready(
function () {
StackExchange.openid.initPostLogin('.new-post-login', 'https%3a%2f%2fmath.stackexchange.com%2fquestions%2f3158927%2fthe-probability-of-bus-a-arriving-before-bus-b%23new-answer', 'question_page');
}
);
Post as a guest
Required, but never shown
Sign up or log in
StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
Sign up or log in
StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
Sign up or log in
StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
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