Discrete Math

3. Given S = {A,B,C,D,E,F}, how many ways are there to perform the following selections? Be sure to show your work.

a. Select three letters from S if repetition is allowed.

b. Select two letters from S if repetition is not allowed.

c. Select four letters from S if repetition is allowed.

d. Select two letters from S such that the first letter is different from the second letter.

e. Select three letters from S such that no two letters in the three selected letters are the same.