A bag contains contains 20 blue marbles, 20 green marbles, and 20 red marbles.
Two marbles are drawn one after the other. no replacement
Using selection (combinations), find the probability of:
(a) drawing two marbles of the same colour
(b) drawing a blue and a green marble
(c) drawing a blue or a green marble
Would the sample space be P(60,2) or C(60,2)