If there are n1 ways to of choosing the first item, n2 ways of choosing the second item after the first item is chosen, n3 ways of choosing the third item after the first two have been ...If there are n1 ways to of choosing the first item, n2 ways of choosing the second item after the first item is chosen, n3 ways of choosing the third item after the first two have been chosen, and so on until there are nk ways of choosing the last item after the earlier choices, then the total number of choices overall is given by
