Combination
GMAT glossary · Verbal Reasoning
Definition
An unordered selection of k items from n items: C(n, k) = (n!)/(k!(n-k)!).
Combination in plain English
A combination is a way of choosing a group where the order you pick doesn't matter, only who ends up in the group. If you're picking 2 friends out of 5 to sit with you, choosing 'Ana then Ben' is the same as 'Ben then Ana,' so it counts once. The formula C(n, k) = (n!)/(k!(n-k)!) does exactly this bookkeeping: it counts all the ordered ways, then divides out the repeats caused by reshuffling the same people. (Here n! means multiply every whole number from 1 up to n.)
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