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"
