I'm having trouble grasping these questions. I understand what it's asking but am not sure how to approach it. Help would be great thanks.

An urn contains n green and m pink marbles, where n and m are positive numbers. a. If two marbles are randomly selected, one at a time without replacement, what is the probability that they are the same colour? b. If a marble is randomly selected and then replaced before the second one is drawn, what is the probability that the withdrawn marbles are the same colour? c. Show that the probability in part (b) is always larger than the one in part (a)Given P(A)=0.28 , P(B)=0.63 , P(AnB)=0.13 Find P(AnB")