I'm doing this old exam in my course and I stumble upon the question:
How many $3$-word sentences can be created from $8$ letters A, $8$ letters B and one of each of the letters C,D,E,F,G when each of these $21$ letters is used exactly once in each sentence?
So after some thinking I reason, if all the letters were unique/distinct the number of 3-word sentences should be the number of bijections. But now I have to somehow deduct that I have 8 A's and 8 B's. Can someone please provide an answer but emphasize on how to account for multiple A's and B's, meaning what would be the difference between all unique letters and with letters repeating.