A is equal to picked fair coin. And then B is equal to 4 out of 6 heads. So in order to figure out the probability that I picked a fair coin given that I got 4 out of 6 heads, I have to know the probability of getting 4 out of 6 heads given that I picked the fair coin times the probability of picking out a fair coin divided by the probability of getting 4 out of 6 heads in general. So this is probably the hardest part to figure out, and we will, along the way, we'll actually probably figure out the top two terms. So what's the probability of B, or the probability of getting 4 out of 6 heads? Let's see what happens. Right when I put my hand into the bag and I pick out a coin, there's a 5 in 10 chance, or 5 in 15 chance, right, there are 15 total coins, that I pick a fair coin.