Probality Problem

Probality Problem

I'm stuck with this can anyone solve this:

A bag contains 12 blue and 8 green counters.
More blue and green counters are added to the bag such that for every blue counter added, two green counters are added.
The probality that a counter taken at random is blue is now 1/2.
How many blue and green counters were added to the bag?

If B blue and 2B green are added to the point where there are equal numbers of each then

12 + B = 8 + 2B

and solving we get B=4 and G=2B=8