Question

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 letter A at least once?

Answer 1

37

Explanation:

AAA 1 way

AAB, AAC, AAD 3 ways each 3*3=9 ways

ABB, ACC, ADD 3 ways each 3*3=9 ways

ABC, ABD, ACD 3*2=6 ways each 3*6=18ways

1+9+9+18 = 37 ways