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Capped doubling payout from coin tosses

Question

Samuel tosses a fair coin until the first tail appears; let nn be the total number of tosses, including that tail. He is then paid min⁡(64,2n)\min(64,2^n) dollars. What is his expected payout?

Short answer

$7. For n≤5n\le 5 the payout 2n2^n has probability 2−n2^{-n}, contributing $1 each, so $5 in total. The cap of $64 applies when n≥6n\ge 6, which has probability 2−52^{-5}, adding 64/32=64/32= $2.

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