164.Three Coins, One Face Showing
★★★conditional probabilityJane Street
A box holds three coins: one with heads on both sides, one with tails on both sides, and one ordinary coin with a head and a tail. You pick a coin at random, toss it, and see heads. What is the probability that the hidden face is also heads?
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Count faces rather than coins. Which faces could you be looking at, and are they equally likely?
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