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. Let's think about how many of those equally likely possibilities involve k heads. Just like we did for the case where we're thinking about 3 heads, we say, well look, the first of those k heads, how many different buckets could it fall into? The first of the k heads could fall into n different buckets.