Use a tree diagram to find the number of ways 3 letters can be chosen from the set {P,Q,R} if order is important and a. if repetition is allowed; b. if no repeats are allowed; c. Find the number of combinations of 3 elements taken 3 at a time. Does this answer differ from that in part (a) or (b)? a. If repetition is allowed, how many ways can 3 letters are chosen from the set {P,Q,R} if order is important?