Hi amidamani13,
You can actually treat this question as a straight Probability question (without using the Combination Formula at all). Here's how:
From the prompt we know about 2 boxes and the marbles within each:
The black box contains
3 red
5 blue
8 total
The white box contains
2 red
3 blue
5 total
We're told to pull 2 marbles from the black box and 1 marble from the white box. We're asked for the probability that ALL marbles pulled will be BLUE.
Let's start with the 1st marble you pull from the black box....
The probability that it's blue is 5/8
Once you pull that first marble, there's 1 fewer blue marble in the black box.
When you grab the 2nd marble from the black box....
The probability that it's blue is 4/7
For the 3rd marble, we pull from the white box.
The probability that it's blue is 3/5
Now, we multiply the results: (5/8)(4/7)(3/5) = 3/14
Final Answer:
GMAT assassins aren't born, they're made,
Rich
In your solution, marbles are pulled one by one. But, if all marbles are pulled at a time, what would be the case, since question does not say to pull one by one?