This calculates C(n,k). How many ways can k be chosen from n?
Hold'em example: How many possible flop combinations are there? You have two hole cards, leaving 50 cards in the deck. We are drawing three cards for the flop, so
C(50,3) = 19,600 possible flop combinations.
[see it]
Keno example: How many possible
keno number combinations are there? There are 80 numbers total and 20 numbers are drawn, so
C(80,20) = 3,535,316,142,212,174,000 possible number combinations. (Yes, 3.5 quintillion!)
[see it]