Basically, choosing one red marble out of everything is 3 / 10 because there are 3 chances of picking the red marble and 10 marbles overall. Picking one blue marble would be 6 / 10, or 3 / 5. You would multiply these two probabilities together because they are independent of each other:
(3 / 10)*(3 / 5) = 9 / 50
The probability is 9 / 50.
