For what it's worth, the COMBIN thing has me confused to the point where I think I'd like to see a formula that doesn't use it.
The combination function is just shorthand notation for a factorial formula.
1) The factorial of N is the number of ways to arrange N distinct elements. It's denoted N!, and it equals N * N-1 * N-2 * ... * 3 * 2 * 1. e.g. 5! = 5*4*3*2*1 = 120. In Excel, the formula is =FACT(n).
2) Suppose you wanted to know how many arrangements there are of M distinct elements from N, where M <= N. There are N ways to pick the first, N-1 ways to pick the 2nd, and so on, down to N-M ways to pick the last, and you multiply those all together. That formula is N! / (N-M)! That's known as the permutation function. So 5p3 = 5*4*3*2*1 / 2*1 = 5*4*3 = 60. In Excel, the formula is =PERMUT(n,m).
3) Now suppose you didn't care how many different arrangements there were of the same M elements, you just want to know how many ways there are to pick them from the larger set of N. For example, you don't care if you have 1 2 3 4 5 or 5 4 3 2 1 - those are the same and you only count those once. Poker hands are a good example - order doesn't matter. Well, we already know how many ways there are to arrange M elements, that's M!. So just divide the permutation function by M!, and now you have N! / (N-M!) / M!, or properly parenthesized, N! / ( (N-M!) * M!). That's the combinations function, and it answers the question "how many ways can I choose M elements from N if I don't care about the order." For this reason, its also commonly spoken as "N choose M". In Excel, the formula is =COMBIN(n,m).
Welcome to combinatorics!
"In my own case, when it seemed to me after a long illness that death was close at hand, I found no little solace in playing constantly at dice."
-- Girolamo Cardano, 1563