20 +30 = 50 total marbles in 1st box
30 are blue
30/50 reduces to 3/5 probability of choosing a blue marble
10+40 = 50 marbles in 2nd box
40 are black
40/50 reduces to 4/5 probability of choosing a black marble
3/5 * 4/*5 = 12/25 probability of choosing both a blue and black marble