120.Re-toss Any Subset
★★★expected valueJane Street
Four fair coins are tossed and you want as many heads as possible. After seeing the result you may re-toss any subset of the coins you choose, once, and you keep whatever the final configuration shows. What is the expected number of heads?
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Decide coin by coin. Re-tossing a head can only hurt, and re-tossing a tail can only help — so treat each coin separately.
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