a bag contains 12blue, 10 green and 6 red and 2 white marbles.

a) If you draw 3 marbles without putting them back, what is the probability that all 3 are blue?
12/30 * 9/27 =0.133?
b) If you draw 2 marbles without putting them back, what is the probability that neither is blue?
18/30 * 16/28=0.34?

(a) You have to multiply for the three draws, and allow for the changing population of blues and the total:

12/30 * 11/29 * 10/28

(b) That would be

18/30 * 17/29