 There are 1024 possible sequences, so the probability of each possible sequence would seem to be \(1/1024\). Of course, only two of them begin with 9 tails in a row, namely the last two. So, once we’ve narrowed things down to a sequence that begins with 9 out of 9 tails, the probability of tails on the 10th toss is \(1/2\), same as heads. More formally, applying the definition conditional probability gives us: