Please help me :)

(2 points) An identification code is to consist of three upper case letters followed by four digits. How many different codes are possible if (a) repetition of letters and numbers is not permitted? (a) (b) repetition of letters and numbers is permitted? (b) (c) repetition of letters is permitted, repetition of numbers is not permitted, and the first three entries must all be the same letter? (c) (d) the first letter must be an A, B, C, or D and repetition of letters and numbers is not permitted