Using the letters a and b, the following two-letter code words can be formed: aa, ab, bb,ba. using the letters a, b, and c, how many different three-letter code words can be formed?
i know that the answer is 27 from the answer sheet, but how? can you list the combinations? !

27

Step-by-step explanation:

Make a tree-diagram for all three-letter code words starting with A. Each path from the top to the bottom contains 3 letters, which is one of the code words beginning with A. There are 9 such code words. Clearly, there are 9 code words starting with B and 9 starting with C. In all, there are 27 code words.

135.91

Step-by-step explanation:

you add them and that is how you find the sum