And sometimes you'll see this formula written like this. n choose k, and they'll write the formula as equal to n factorial over n minus k factorial. And also in the denominator, k factorial. And this is a general formula that if you have n things and you want to find out all of the possible ways you can pick k things from those n things and you don't care about the order. All you care is about which k things you picked. You don't care about the order in which you picked those k things. So that's what we did here.