2015 AMC 10B Problem 18

A coin ends as tails if and only if it has $3$ flips that are tails, which happens with probability $\frac 18.$ Thus, the probability of any coin being heads is $\frac 78 .$ Since each of the $64$ coins ends as heads with probability $\frac78,$ linearity of expectation gives the expected number of coins that are now heads: $$64 \cdot \dfrac 78 = 56.$$ Thus, the correct answer is D .