Who keeps the ball in a five-person circle
Question
Five people, including you, sit in a circle, and you hold a ball at the start. Each second, whoever holds the ball picks one of three actions uniformly at random: pass it to the neighbour on the left, pass it to the neighbour on the right, or keep it. Play ends as soon as a holder decides to keep it. Find the chance that the game finishes with the ball in your hands.
Short answer
. By symmetry only three states matter: you hold it (), a neighbour holds it (), or one of the two far players holds it (). Then , , . These give , and finally .
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Open the worked solutionLessons behind this question
- Sample Spaces, Events, and the Probability Axioms
- Conditional Probability, Total Probability, and Bayes' Rule
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