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To respin or not with two randomly loaded chambers

Question

Two bullets are loaded into two randomly chosen chambers of a six-shot revolver, the cylinder is spun, and your opponent pulls the trigger and survives. It is now your turn. Would you rather pull the trigger straight away (moving to the next chamber) or spin the cylinder first?

Short answer

Spin. A fresh spin fires with probability 26=13\tfrac26=\tfrac13. Without spinning, you know the current chamber is empty, so the next chamber is one of the other five, two of which are loaded: probability 25>13\tfrac25>\tfrac13. (If the two bullets were in adjacent chambers instead, not spinning would be safer at 14\tfrac14.)

Adapted from Zhou, A Practical Guide to Quantitative Finance Interviews, sec. 4.3 Conditional Probability and Bayes' Formula, 'Russian roulette III', p.81; question and answer rewritten by SelfMadeQuant.com.

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