A box contains 20 red marbles and 30 blue marbles. A second box contains 10 white marbles and 40 black marbles. If you choose one marble from each box without looking, what is the probability that you get a blue marble and a black marble?
The total number of marbles in the first box is 20 + 30 = 50.
The total number of marbles in the second box is 10 + 40 = 50.
The probability of choosing a blue marble from the first box is 30/50 = 3/5.
The probability of choosing a black marble from the second box is 40/50 = 4/5.
Therefore, the probability of choosing a blue marble and a black marble is (3/5) * (4/5) = 12/25. Answer: \boxed{\frac{12}{25}}.