The number of ways to choose k items from n items is C(n,k) = n! / (k!(n-k)!).
This formula is also written as "n choose k" or using binomial coefficient notation. All three names refer to the same value.
Example: C(5,2) = 5! / (2! × 3!) = 120 / (2 × 6) = 120 / 12 = 10. There are 10 ways to choose 2 items from 5.