If you have 21tiles and there are twice as many blue than red and twice as many red than green how many red green and blue are there.
r + b + g = 21
b = 2r
r = 2g
sub back into the first
r + 2r + r/2 = 21
3.5r = 21
r = 6
so 6 red
12 blue and
3 green
check: do they add up to 21 ? YES
are there twice as many blue than red ? YES
are there twice as many red than green ? YES