Brain Teasers & Mental Math
What is the expected number of fair-coin flips to get two heads IN A ROW?
Model answer
6 flips.
Set up states. Let E₀ = expected flips from scratch, E₁ = expected flips holding one head. From E₁: flip once — heads (prob 1/2) finishes; tails restarts: E₁ = 1 + (1/2)(0) + (1/2)E₀. From…
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