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. COMBINA returns the number of unique ways to choose these items, where the order of choosing is irrelevant, and repetition of items is allowed. For example if there are 3 items A, B and C in a set, you can choose 2 items in 6 different ways, namely AA, AB, AC, BB, BC and CC; you can choose 3 items in 10 different ways, namely AAA, AAB, AAC, ABB, ABC, ACC, BBB, BBC, BCC, CCC. COMBINA implements the formula:(n+k-1)!/(k!(n-1)!)