Odd coin problem
On average, how many times do you need to flip a fair coin before you have seen a run of an odd number of heads, followed by a tail?
i.e. you only stop flipping when you have seen any of 1H1T or 3H1T or 5H1T or … all the way to infinity. e.g. HT, THT, HHHT, HHTTTHHHT are possible runs (you stop on the last T).