ii. How many three-letter codes are there using letters A, B, C, and D if no letter can be repeated?

Answer 1

If you assume that the order of the letter matters (i.e. ABC is different from CBA), then the formula is:

N!/(N-R)! where N is the total number of letters and R is the number of letters in the code. And we get:

4!/(4-3)! = 4 x 3 x 2 = 24