The Hurried Duelers brainteaser

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This question is similar as this other one asked in the forum, but I am trying to give it a different twist. Unfortunately, I am not getting to the same answer, so there might be something wrong in my math.
The problem in this case is the following:
Duels in the town of Discretion are rarely fatal. There, each contestant comes at a random moment between 5 a.m. and 6 a.m. on the appointed day and leaves exactly 5 minutes later, honor served, unless his opponent arrives within the time interval and then they fight. What fraction of duels lead to violence?
My solution is the following. Let X and Y be uniformly distributed random variables on $[0,60]$, each corresponding to the time of arrival of each dueler. The desired probability is
$$
P(X + 5 < Y) + P(Y+5<X)
$$
which, by symmetry, equals $2,P(X+5<Y)$. I have done the following:
$$
2P(X+5<Y) = 2iint_X<Y-5f_X,Y(x,y),dx,dy = 2int_5^60int_0^y-5left(frac160right)^2,dx,dy
$$
But this does not lead to the desired probability $frac16$. Is perhaps my approach wrong?
probability probability-theory uniform-distribution
add a comment |Â
up vote
2
down vote
favorite
This question is similar as this other one asked in the forum, but I am trying to give it a different twist. Unfortunately, I am not getting to the same answer, so there might be something wrong in my math.
The problem in this case is the following:
Duels in the town of Discretion are rarely fatal. There, each contestant comes at a random moment between 5 a.m. and 6 a.m. on the appointed day and leaves exactly 5 minutes later, honor served, unless his opponent arrives within the time interval and then they fight. What fraction of duels lead to violence?
My solution is the following. Let X and Y be uniformly distributed random variables on $[0,60]$, each corresponding to the time of arrival of each dueler. The desired probability is
$$
P(X + 5 < Y) + P(Y+5<X)
$$
which, by symmetry, equals $2,P(X+5<Y)$. I have done the following:
$$
2P(X+5<Y) = 2iint_X<Y-5f_X,Y(x,y),dx,dy = 2int_5^60int_0^y-5left(frac160right)^2,dx,dy
$$
But this does not lead to the desired probability $frac16$. Is perhaps my approach wrong?
probability probability-theory uniform-distribution
Check the probability you wrote down in the first equation. The given event is $(X < Y < X + 5text or Y < X < Y + 5)$, which isn't quite what you wrote.
â anomaly
Feb 11 '15 at 14:49
How many duelers come within that 1 hour?
â ghosts_in_the_code
Feb 11 '15 at 14:50
@Iliana: What you wrote corresponds to the event where $X$ leaves before $Y$ arrives or vice versa. (The events are disjoint and so their probabilities add). That works, but you should have $1 - cdots$ term in there.
â anomaly
Feb 11 '15 at 15:12
add a comment |Â
up vote
2
down vote
favorite
up vote
2
down vote
favorite
This question is similar as this other one asked in the forum, but I am trying to give it a different twist. Unfortunately, I am not getting to the same answer, so there might be something wrong in my math.
The problem in this case is the following:
Duels in the town of Discretion are rarely fatal. There, each contestant comes at a random moment between 5 a.m. and 6 a.m. on the appointed day and leaves exactly 5 minutes later, honor served, unless his opponent arrives within the time interval and then they fight. What fraction of duels lead to violence?
My solution is the following. Let X and Y be uniformly distributed random variables on $[0,60]$, each corresponding to the time of arrival of each dueler. The desired probability is
$$
P(X + 5 < Y) + P(Y+5<X)
$$
which, by symmetry, equals $2,P(X+5<Y)$. I have done the following:
$$
2P(X+5<Y) = 2iint_X<Y-5f_X,Y(x,y),dx,dy = 2int_5^60int_0^y-5left(frac160right)^2,dx,dy
$$
But this does not lead to the desired probability $frac16$. Is perhaps my approach wrong?
probability probability-theory uniform-distribution
This question is similar as this other one asked in the forum, but I am trying to give it a different twist. Unfortunately, I am not getting to the same answer, so there might be something wrong in my math.
The problem in this case is the following:
Duels in the town of Discretion are rarely fatal. There, each contestant comes at a random moment between 5 a.m. and 6 a.m. on the appointed day and leaves exactly 5 minutes later, honor served, unless his opponent arrives within the time interval and then they fight. What fraction of duels lead to violence?
My solution is the following. Let X and Y be uniformly distributed random variables on $[0,60]$, each corresponding to the time of arrival of each dueler. The desired probability is
$$
P(X + 5 < Y) + P(Y+5<X)
$$
which, by symmetry, equals $2,P(X+5<Y)$. I have done the following:
$$
2P(X+5<Y) = 2iint_X<Y-5f_X,Y(x,y),dx,dy = 2int_5^60int_0^y-5left(frac160right)^2,dx,dy
$$
But this does not lead to the desired probability $frac16$. Is perhaps my approach wrong?
probability probability-theory uniform-distribution
edited Apr 13 '17 at 12:20
Communityâ¦
1
1
asked Feb 11 '15 at 14:34
Iliana
1507
1507
Check the probability you wrote down in the first equation. The given event is $(X < Y < X + 5text or Y < X < Y + 5)$, which isn't quite what you wrote.
â anomaly
Feb 11 '15 at 14:49
How many duelers come within that 1 hour?
â ghosts_in_the_code
Feb 11 '15 at 14:50
@Iliana: What you wrote corresponds to the event where $X$ leaves before $Y$ arrives or vice versa. (The events are disjoint and so their probabilities add). That works, but you should have $1 - cdots$ term in there.
â anomaly
Feb 11 '15 at 15:12
add a comment |Â
Check the probability you wrote down in the first equation. The given event is $(X < Y < X + 5text or Y < X < Y + 5)$, which isn't quite what you wrote.
â anomaly
Feb 11 '15 at 14:49
How many duelers come within that 1 hour?
â ghosts_in_the_code
Feb 11 '15 at 14:50
@Iliana: What you wrote corresponds to the event where $X$ leaves before $Y$ arrives or vice versa. (The events are disjoint and so their probabilities add). That works, but you should have $1 - cdots$ term in there.
â anomaly
Feb 11 '15 at 15:12
Check the probability you wrote down in the first equation. The given event is $(X < Y < X + 5text or Y < X < Y + 5)$, which isn't quite what you wrote.
â anomaly
Feb 11 '15 at 14:49
Check the probability you wrote down in the first equation. The given event is $(X < Y < X + 5text or Y < X < Y + 5)$, which isn't quite what you wrote.
â anomaly
Feb 11 '15 at 14:49
How many duelers come within that 1 hour?
â ghosts_in_the_code
Feb 11 '15 at 14:50
How many duelers come within that 1 hour?
â ghosts_in_the_code
Feb 11 '15 at 14:50
@Iliana: What you wrote corresponds to the event where $X$ leaves before $Y$ arrives or vice versa. (The events are disjoint and so their probabilities add). That works, but you should have $1 - cdots$ term in there.
â anomaly
Feb 11 '15 at 15:12
@Iliana: What you wrote corresponds to the event where $X$ leaves before $Y$ arrives or vice versa. (The events are disjoint and so their probabilities add). That works, but you should have $1 - cdots$ term in there.
â anomaly
Feb 11 '15 at 15:12
add a comment |Â
3 Answers
3
active
oldest
votes
up vote
1
down vote
accepted
Your calculation should be $$1- displaystyle 2int_5^60int_0^y-5left(frac160right)^2,dx,dy$$ which would be $1-frac55^260^2 approx 0.1597$ or slightly less than $frac16$. I am not sure you have multiplied by $2$ or subtracted from $1$. Perhaps this picture helps

@ Henry why not to solve it using just simple geometry where the searched probability is the difference between the surface of the whole square - 2 * the surface of the white lower triangle $1-2 frac12 (frac1112)^2=frac23144 approx frac16$. Isn't the double integral over complicating the solution?
â Michal
Dec 10 '16 at 23:11
add a comment |Â
up vote
1
down vote
The condition $X+5 < Y$ actually guarantees there will not be a duel.
Similarly for $Y+5<X$.
If you compute your integral correctly, you need to subtract twice the
integral from $1$ to get the probability that there will be a duel.
It looks like somewhere you lost a factor of $2$.
The integral itself should be at least $0.42$ and double it should be at least $0.84$.
The probability of a duel turns out to be less than $frac 16$.
add a comment |Â
up vote
-3
down vote
For the violence to occur next man should come in $5$ mins after the first one so the probability of that is $5/60$. but these man can interchange the position so answer will be $2*5/60=1/6$.
add a comment |Â
3 Answers
3
active
oldest
votes
3 Answers
3
active
oldest
votes
active
oldest
votes
active
oldest
votes
up vote
1
down vote
accepted
Your calculation should be $$1- displaystyle 2int_5^60int_0^y-5left(frac160right)^2,dx,dy$$ which would be $1-frac55^260^2 approx 0.1597$ or slightly less than $frac16$. I am not sure you have multiplied by $2$ or subtracted from $1$. Perhaps this picture helps

@ Henry why not to solve it using just simple geometry where the searched probability is the difference between the surface of the whole square - 2 * the surface of the white lower triangle $1-2 frac12 (frac1112)^2=frac23144 approx frac16$. Isn't the double integral over complicating the solution?
â Michal
Dec 10 '16 at 23:11
add a comment |Â
up vote
1
down vote
accepted
Your calculation should be $$1- displaystyle 2int_5^60int_0^y-5left(frac160right)^2,dx,dy$$ which would be $1-frac55^260^2 approx 0.1597$ or slightly less than $frac16$. I am not sure you have multiplied by $2$ or subtracted from $1$. Perhaps this picture helps

@ Henry why not to solve it using just simple geometry where the searched probability is the difference between the surface of the whole square - 2 * the surface of the white lower triangle $1-2 frac12 (frac1112)^2=frac23144 approx frac16$. Isn't the double integral over complicating the solution?
â Michal
Dec 10 '16 at 23:11
add a comment |Â
up vote
1
down vote
accepted
up vote
1
down vote
accepted
Your calculation should be $$1- displaystyle 2int_5^60int_0^y-5left(frac160right)^2,dx,dy$$ which would be $1-frac55^260^2 approx 0.1597$ or slightly less than $frac16$. I am not sure you have multiplied by $2$ or subtracted from $1$. Perhaps this picture helps

Your calculation should be $$1- displaystyle 2int_5^60int_0^y-5left(frac160right)^2,dx,dy$$ which would be $1-frac55^260^2 approx 0.1597$ or slightly less than $frac16$. I am not sure you have multiplied by $2$ or subtracted from $1$. Perhaps this picture helps

edited Feb 11 '15 at 15:04
answered Feb 11 '15 at 14:59
Henry
93.5k471149
93.5k471149
@ Henry why not to solve it using just simple geometry where the searched probability is the difference between the surface of the whole square - 2 * the surface of the white lower triangle $1-2 frac12 (frac1112)^2=frac23144 approx frac16$. Isn't the double integral over complicating the solution?
â Michal
Dec 10 '16 at 23:11
add a comment |Â
@ Henry why not to solve it using just simple geometry where the searched probability is the difference between the surface of the whole square - 2 * the surface of the white lower triangle $1-2 frac12 (frac1112)^2=frac23144 approx frac16$. Isn't the double integral over complicating the solution?
â Michal
Dec 10 '16 at 23:11
@ Henry why not to solve it using just simple geometry where the searched probability is the difference between the surface of the whole square - 2 * the surface of the white lower triangle $1-2 frac12 (frac1112)^2=frac23144 approx frac16$. Isn't the double integral over complicating the solution?
â Michal
Dec 10 '16 at 23:11
@ Henry why not to solve it using just simple geometry where the searched probability is the difference between the surface of the whole square - 2 * the surface of the white lower triangle $1-2 frac12 (frac1112)^2=frac23144 approx frac16$. Isn't the double integral over complicating the solution?
â Michal
Dec 10 '16 at 23:11
add a comment |Â
up vote
1
down vote
The condition $X+5 < Y$ actually guarantees there will not be a duel.
Similarly for $Y+5<X$.
If you compute your integral correctly, you need to subtract twice the
integral from $1$ to get the probability that there will be a duel.
It looks like somewhere you lost a factor of $2$.
The integral itself should be at least $0.42$ and double it should be at least $0.84$.
The probability of a duel turns out to be less than $frac 16$.
add a comment |Â
up vote
1
down vote
The condition $X+5 < Y$ actually guarantees there will not be a duel.
Similarly for $Y+5<X$.
If you compute your integral correctly, you need to subtract twice the
integral from $1$ to get the probability that there will be a duel.
It looks like somewhere you lost a factor of $2$.
The integral itself should be at least $0.42$ and double it should be at least $0.84$.
The probability of a duel turns out to be less than $frac 16$.
add a comment |Â
up vote
1
down vote
up vote
1
down vote
The condition $X+5 < Y$ actually guarantees there will not be a duel.
Similarly for $Y+5<X$.
If you compute your integral correctly, you need to subtract twice the
integral from $1$ to get the probability that there will be a duel.
It looks like somewhere you lost a factor of $2$.
The integral itself should be at least $0.42$ and double it should be at least $0.84$.
The probability of a duel turns out to be less than $frac 16$.
The condition $X+5 < Y$ actually guarantees there will not be a duel.
Similarly for $Y+5<X$.
If you compute your integral correctly, you need to subtract twice the
integral from $1$ to get the probability that there will be a duel.
It looks like somewhere you lost a factor of $2$.
The integral itself should be at least $0.42$ and double it should be at least $0.84$.
The probability of a duel turns out to be less than $frac 16$.
answered Feb 11 '15 at 14:59
David K
48.8k340109
48.8k340109
add a comment |Â
add a comment |Â
up vote
-3
down vote
For the violence to occur next man should come in $5$ mins after the first one so the probability of that is $5/60$. but these man can interchange the position so answer will be $2*5/60=1/6$.
add a comment |Â
up vote
-3
down vote
For the violence to occur next man should come in $5$ mins after the first one so the probability of that is $5/60$. but these man can interchange the position so answer will be $2*5/60=1/6$.
add a comment |Â
up vote
-3
down vote
up vote
-3
down vote
For the violence to occur next man should come in $5$ mins after the first one so the probability of that is $5/60$. but these man can interchange the position so answer will be $2*5/60=1/6$.
For the violence to occur next man should come in $5$ mins after the first one so the probability of that is $5/60$. but these man can interchange the position so answer will be $2*5/60=1/6$.
edited Aug 19 at 6:03
user529760
answered Aug 19 at 3:56
Gaurav
1
1
add a comment |Â
add a comment |Â
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Check the probability you wrote down in the first equation. The given event is $(X < Y < X + 5text or Y < X < Y + 5)$, which isn't quite what you wrote.
â anomaly
Feb 11 '15 at 14:49
How many duelers come within that 1 hour?
â ghosts_in_the_code
Feb 11 '15 at 14:50
@Iliana: What you wrote corresponds to the event where $X$ leaves before $Y$ arrives or vice versa. (The events are disjoint and so their probabilities add). That works, but you should have $1 - cdots$ term in there.
â anomaly
Feb 11 '15 at 15:12