409.Rolling to an Even Sum
★★★expected valueCh. 5 — Multivariate Distributions and Moments444+ Problems in Probability
A fair 6-sided die is rolled until the running sum of all upfaces is even. Find the expected number of rolls.
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After the first roll, either you're done or you need the sum's parity to flip — and each roll flips it with probability 1/2.
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