# Combinations Combinations tell us how many possible ways are there to form bag of non-repeating elements (where order is not important). This is in contrast to [[Permutations]] where order is relevant. The formula is (n is the size of the set, k is the number elements you will permute). It is also known as the binomial coefficient. $ _nC_k = \binom n k = \frac{n!}{k!(n-k)!} = \frac{_nP_k}{k!} $ Combinations are pronounced as 'n choose k'. Note that $\binom n k = \binom n {n-k}$ because the number of possible ways to select k elements equals the number of possible ways to deselect (n-k) elements.