There are 12 marbles in a bag, and the marbles are either yellow or green. Two marbles will be randomly picked from the bag, without replacing the first one picked. The probability that both marbles will be yellow is 5 33 . How many YELLOW marbles are in the bag?

7 green, 5 yellow. Just worked backwards. C(12,2) = 66, so probability given is also 10/66 

If there are x yellow marbles, then C(x,2)•C(12-x,0)/C(12,2) = 10/66. Thus C(x, 2) = 10, so x = 5 = yellows. So greens = 12–5 = 7