Given a set of 5 letters, in how many different ways can we choose 3 letters and then arrange them in a different order a long a line?
Ways to choose 3 of 5 letters
= C(5,3) = 10
Then each of these can be arranged in 3! or 6 ways
So.......
= C(5,3) = 10
Then each of these can be arranged in 3! or 6 ways
So.......