Returns the number of combinations of a subset of items.
- n is the number of items in the set.
- k is the number of items to choose from the set.
- COMBIN returns the number of ways to choose these items. For example if there are 3 items A, B and C in a set, you can choose 2 items in 3 different ways, namely AB, AC and BC.
- COMBIN implements the formula:
- returns 3.