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 . 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 . (If the two bullets were in adjacent chambers instead, not spinning would be safer at .)
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.
See the hint and the full worked solution in the question bank (free account).
Open the worked solutionLessons behind this question
- Conditional Probability, Total Probability, and Bayes' Rule
- Worked Examples — Probability Trees, Monty Hall, and Bayesian Updating
- Sample Spaces, Events, and the Probability Axioms
Start with a free lesson: Probability Fundamentals is part of a free topic. Create a free account, no card needed, to read it in full with worked solutions.
Read it free