Find the number of ways to choose r items from a set of n, where order does not matter.
Counts unordered selections — choosing a team of 3 from 10 people is a combination problem.
nCr: n! ÷ (r! × (n−r)!)
Why is nCr always smaller than nPr?
Combinations do not care about the order of the same group, so many different permutations count as just one combination. Dividing by r! removes these repeated orderings.