The letters ABCDEFGH are to be used to form strings of length 4. How many strings contain the letters A and B if repetitions are not allowed?
Here is my answer:
The total number of possible combinations are $8×7×6×5$
The total number of combinations if A and B are not allowed is $6×5×4×3$
So the answer is the difference of the two, which is $(8×7×6×5) - (6×5×4×3) = 1320$
Please tell me if I am correct and if not, point me in the right direction.