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Distracted by Thoughts of Greater Magnitude

Source:

March 16, 2010
probability

Problem Statement

Terence Tao is playing rock-paper-scissors. Because his mental energy is focused on solving the twin primes conjecture, he uses the following very simple strategy:    ·He plays rock first.    ·On each subsequent turn, he plays a different move than the previous one, each with probability ½. What is the probability that his 5th move will be rock?