Bayes is playing Russian roulette

There was (once again) a nice puzzle inhttp://www.futilitycloset.com/. Bayes and a good friend are playing Russian roulette. The revolver has six chambers. He puts two bullets in two adjacent chambers, spin the cylinder, hold the gun to his friend’s head, and pull the trigger. It clicks. So it is now Bayes’s turn: he can choose either to spin the cylinder again or leave it as it is. Which is better? Hopefully, Bayes knows his theorem: if he does spin it, the probability of getting killed is 2 out of 6 (four empty chambers out of six), but if he does not, since his friend is still alive, then the hammer should be next to one of the four cylinders in red, below


So here, there is 3 chance out of 4 to survive, i.e. the probability of getting killed is 1 out of 4 (while it was 1 out of 3 when spinning). So Bayes should not spin. And as always, it is possible to see it is a more general result: more generally, in a revolver with http://freakonometrics.blog.free.fr/public/perso5/bullet01.gif chambers, it there are http://freakonometrics.blog.free.fr/public/perso5/bullet02.gif bullets in http://freakonometrics.blog.free.fr/public/perso5/bullet02.gif adjacent chambers,  if the first player survives, the probability of getting killed is k over http://freakonometrics.blog.free.fr/public/perso5/bullet01.gif, when spinning, while it would be 1 over http://freakonometrics.blog.free.fr/public/perso5/bullet03.gif if we don’t. Not spinning is better if and only if

http://freakonometrics.blog.free.fr/public/perso5/bullet04.gif

i.e.

http://freakonometrics.blog.free.fr/public/perso5/bullet05.gif

So you’d better not spin, unless there was one bullet in the revolver, i.e. http://freakonometrics.blog.free.fr/public/perso5/bullet06.gif… or http://freakonometrics.blog.free.fr/public/perso5/bullet07.gif (in that case, it might not be a good idea actually to play the game).


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