The probability of 2 to the n equally likely possibilities, and how many of those possibilities result in exactly k heads? We just figured that out during this video. That's the number of possibilities. n factorial over k factorial times n minus k factorial. It probably is an okay idea to memorize this, but I'll just tell you frankly, the only reason why I still know how to do this 20 years after first seeing it, or whenever I first saw it, is that I actually just like to reason through it every time. I like to just reason through, okay, I've got 5 flips, 3 of them need to be heads, the first of those heads can be in 5 different buckets, the next in 4 different buckets, the next one in 3 different buckets, and then of course I don't want to differentiate between all of the different ways that I can rearrange 3 different things, so I have to make sure that I divide it by 3 factorial, by 3 times 2 times 1. I want to make sure that I divide it by all of the different ways that I can arrange 3 different things.