Four heads in a row

What is the expected number of fair-coin flips to get 4 heads in a row for the first time?

Show hints (2)+
  1. Relate the expectation for n to the one for n−1.
  2. En=2En−1+2=2n+1−2E_n = 2E_{n-1} + 2 = 2^{n+1}-2.

Answer

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30

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