Chapter 8.2, Problem 16E

(a)

To determine

The number of ways in which 2 letters can be chosen from the set $\left\{\text{L,M,N,P}\right\}$ if order is important and repetition is allowed, by using a tree diagram.

(b)

To determine

The number of ways in which 2 letters can be chosen from the set $\left\{\text{L,M,N,P}\right\}$ if order is important and repetition is not allowed, by using a tree diagram.

(c)

To determine

The number of combinations of 4 elements taken 2 at a time.