Thanks for this! I was confused myself. http://www.wolframalpha.com/input/?i=n%21%2F%28%28n%2F2%29%21%282^%28n%2F2%29%29%29%2C+n%3D40 http://www.wolframalpha.c

Thanks for this! I was confused myself. http://www.wolframalpha.com/input/?i=n%21%2F%28%28n%2F2%29%21%282^%28n%2F2%29%29%29%2C+n%3D40 http://www.wolframalpha.com/input/?i=n%21%2F%28%28n%2F2%29%2... seems to give 319830986772877770815625 It can also be done by thinking 39 * 37 * 35 * ... * 1 which also yields the same number. In that way, it is like selecting 1 person, who then has 39 people remaining, then selecting another, who has 37 left to choose from and so on. I was super confused at first why it wasn't just 40 choose 2.