Abox 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 probability of you picking out a blue marble would be 30/50 and the probability of you picking out a black marble would be 40/50.
To find out the probability of both you just add them together so 70/100.
I hope this helps. 

I think it's 12/25. 
The first box has 20 red and 30 blue, which equals 50 marbles total.
The probability of pulling a blue marble out is 30/50, or 3/5.
The second box has 10 white and 40 black, which, again, equals 50 marbles total. The probability of pulling a black marble out is 40/50, or 4/5. 
3/5*4/5=12/25=4/5.
Hope this helps =)