And if you look at it, based on our definition of B and A, this is the probability of B given A. B is 4 out of 6 heads, given a fair coin. Fair enough. So let's figure out the probability of, because there's two ways of getting 4 out of 6 heads. One, that we picked a fair coin, and then times 15 out of 64, and then there's a probability that we picked an unfair coin. So what's the probability of the unfair coin? Of getting 4 out of 6 heads, given the unfair coin?