Combination

A combination is a selection of items in which order is irrelevant. The number of ways to choose r items from n distinct items is nCr = n! ÷ (r! × (n − r)!), read as “n choose r”. Combinations count groupings — lottery tickets, committees, or pizza toppings — and are smaller than the corresponding permutations because each unordered group is counted once.

Example: Choosing 2 toppings from 4 gives 4C2 = 4! ÷ (2! × 2!) = 6 combinations.