In general, if you wanted to figure out the number of ways to stick 2 things in 5 chairs and you don't care about differentiating between those 2 things, you're going to have this expression right over here, which is the same thing as this right over here. You're going to have n factorial over n minus k factorial. Then you're going to divide it by this expression right over here, which we already said is the same thing as k factorial. You're also going to divide it by k factorial. Then you have a generalized way of figuring out the number of ways you can stick 2 things, or the number of ways you can stick k things in n different buckets, k heads in n different flips. This is actually a generalized formula for binomial coefficients. Another way to write this is the number of ways, given that you have n buckets, you can put k things in them without having to differentiate it.