A bag contains 10 marbles, each either red or blue.  If two marbles are randomly selected, the chance that they are both blue is 2/15.  How many red marbles are in the bag?

 

 

Using b as the number of blue marbles in the bag, the probability of two blues is:

 

 

Factoring:

 

 

We cannot have a negative number of marbles so b = 4.  There are 10 marbles total so 10 – 4 = 6, there are 6 red marbles.