How many 3-letter words can we make from the letters A, B, C, and D, if we are allowed to repeat letters, and we must use the letters A and B at least once?

Possibilities

ABA = Choose any one of three positions for B = 3

ABB = Choose any one of three positions for A = 3

ABC = 3! ways to arrange this = 6

ABD = 3! ways to arrange this = 6

18 possible "words"