What I want to do is figure out the probability of getting k heads in n flips of the fair coin. The first thing to think about is how many possibilities there are. There's going to be n flips, so literally there's first flip, second flip, third flip, all the way to the nth flip. This is a fair coin. Each of these has two equally likely possibilities. The total number of possibilities is going to be 2 times 2 times 2 n times. This is going to be equal to 2 to the nth possibilities.