The answer is:
R = 3
W = 7
B=11

To solve it I just wrote the info as equations and by solving them I came up with the following two equations:
18xy = 441-21x
28xy=441+21y
(I changed R to x, W to y and B to z)

I solved one of the above to x and substitute in the other. I then came up with two solutions, one was not an integer which is not acceptable and so the result was the other one.

If you want a more detailed solution I can provide it.