Redner, S.
We use the backward Kolmogorov equation approach to understand the apparently paradoxical feature that the mean waiting time to encounter distinct fixed-length sequences of heads and tails upon repeated fair coin flips can be different. For sequences of length 2, the mean time until the sequence HH (heads-heads) appears equals 6, while the waiting time for the sequence HT (heads-tails) equals 4. We give complete results for the waiting times of sequences of lengths 3, 4, and 5; the extension to longer sequences is straightforward (albeit more tedious). We also derive moment generating functions, from which any moment of the mean waiting time for specific sequences can be found. Finally, we compute the mean waiting times T2nH for 2n heads in a row, as well as the moment generating function for this sequence, and Tn(HT) for n alternating heads and tails. For large n, T2nH similar to 3Tn(HT). Thus distinct sequences of coin flips of the same length can have very different mean waiting times.