The Hurried Duelers brainteaser

The name of the pictureThe name of the pictureThe name of the pictureClash Royale CLAN TAG#URR8PPP











up vote
2
down vote

favorite
1












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?







share|cite|improve this question






















  • 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.
    – anomaly
    Feb 11 '15 at 14:49










  • How many duelers come within that 1 hour?
    – 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.
    – anomaly
    Feb 11 '15 at 15:12














up vote
2
down vote

favorite
1












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?







share|cite|improve this question






















  • 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.
    – anomaly
    Feb 11 '15 at 14:49










  • How many duelers come within that 1 hour?
    – 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.
    – anomaly
    Feb 11 '15 at 15:12












up vote
2
down vote

favorite
1









up vote
2
down vote

favorite
1






1





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?







share|cite|improve this question














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?









share|cite|improve this question













share|cite|improve this question




share|cite|improve this question








edited Apr 13 '17 at 12:20









Community♦

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.
    – anomaly
    Feb 11 '15 at 14:49










  • How many duelers come within that 1 hour?
    – 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.
    – 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.
    – anomaly
    Feb 11 '15 at 14:49










  • How many duelers come within that 1 hour?
    – 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.
    – 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.
– 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.
– anomaly
Feb 11 '15 at 14:49












How many duelers come within that 1 hour?
– ghosts_in_the_code
Feb 11 '15 at 14:50




How many duelers come within that 1 hour?
– 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.
– 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.
– anomaly
Feb 11 '15 at 15:12










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



enter image description here






share|cite|improve this answer






















  • @ 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?
    – Michal
    Dec 10 '16 at 23:11


















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 $frac 16$.






share|cite|improve this answer



























    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 $2*5/60=1/6$.






    share|cite|improve this answer






















      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',
      convertImagesToLinks: true,
      noModals: false,
      showLowRepImageUploadWarning: true,
      reputationToPostImages: 10,
      bindNavPrevention: true,
      postfix: "",
      noCode: true, onDemand: true,
      discardSelector: ".discard-answer"
      ,immediatelyShowMarkdownHelp:true
      );



      );








       

      draft saved


      draft discarded


















      StackExchange.ready(
      function ()
      StackExchange.openid.initPostLogin('.new-post-login', 'https%3a%2f%2fmath.stackexchange.com%2fquestions%2f1143590%2fthe-hurried-duelers-brainteaser%23new-answer', 'question_page');

      );

      Post as a guest






























      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



      enter image description here






      share|cite|improve this answer






















      • @ 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?
        – Michal
        Dec 10 '16 at 23:11















      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



      enter image description here






      share|cite|improve this answer






















      • @ 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?
        – Michal
        Dec 10 '16 at 23:11













      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



      enter image description here






      share|cite|improve this answer














      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



      enter image description here







      share|cite|improve this answer














      share|cite|improve this answer



      share|cite|improve this answer








      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?
        – 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?
        – 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?
      – 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?
      – Michal
      Dec 10 '16 at 23:11











      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 $frac 16$.






      share|cite|improve this answer
























        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 $frac 16$.






        share|cite|improve this answer






















          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 $frac 16$.






          share|cite|improve this answer












          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 $frac 16$.







          share|cite|improve this answer












          share|cite|improve this answer



          share|cite|improve this answer










          answered Feb 11 '15 at 14:59









          David K

          48.8k340109




          48.8k340109




















              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 $2*5/60=1/6$.






              share|cite|improve this answer


























                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 $2*5/60=1/6$.






                share|cite|improve this answer
























                  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 $2*5/60=1/6$.






                  share|cite|improve this answer














                  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 $2*5/60=1/6$.







                  share|cite|improve this answer














                  share|cite|improve this answer



                  share|cite|improve this answer








                  edited Aug 19 at 6:03







                  user529760

















                  answered Aug 19 at 3:56









                  Gaurav

                  1




                  1






















                       

                      draft saved


                      draft discarded


























                       


                      draft saved


                      draft discarded














                      StackExchange.ready(
                      function ()
                      StackExchange.openid.initPostLogin('.new-post-login', 'https%3a%2f%2fmath.stackexchange.com%2fquestions%2f1143590%2fthe-hurried-duelers-brainteaser%23new-answer', 'question_page');

                      );

                      Post as a guest













































































                      這個網誌中的熱門文章

                      tkz-euclide: tkzDrawCircle[R] not working

                      How to combine Bézier curves to a surface?

                      1st Magritte Awards