Unknown variable in Probability question

lisbon_lions

Hey,

I have been doing some probability and we have moved onto conditional probability with mutual exclusivity, independence and such like. Im pretty ok when I have the variables to the left of the = sign. However my reasoning breaks down when i have one variable and an answer but not the 2nd variable.

For e.g.

A bag contains 5 black and x blue marbles. One marble is drawn at random without replacement and then another drawn. If the probability of drawing 2 blue marbles is 1/6, how many blue marbles are in the bag?
The answer is

4

but I dont know a good routine to get to that - can anybody help with a way to achieve this?

Hope this makes sense, let me know if you need more info.
Cheers.

Pherekydes

The probability of the first marble being blue is x/(x+5)

Can you write an expression for the probability that the second one is also blue? If you can, multiply these two expressions together and set equal to 1/6 and solve for x.