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Capped doubling payout from tossing until heads

Question

Ken tosses a fair coin repeatedly and stops at the first head. If that happens on toss kk, he is paid min⁡(2k,64)\min(2^k,64) dollars. What is his expected payout?

Short answer

$7. For k=1,…,6k=1,\dots,6 each term contributes 2−k⋅2k=12^{-k}\cdot2^k=1. For k≥7k\ge7 the payout is capped at 6464, and P(k≥7)=2−6P(k\ge7)=2^{-6}, contributing 11. Total 6+1=76+1=7.

Adapted from QuantQuestions.io; question and answer rewritten by SelfMadeQuant.com.

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