I have a bag with blue marbles and yellow marbles in it. At the moment, the ratio of blue marbles to yellow marbles is 8:5. If I remove 12 blue marbles and add 21 yellow marbles, the ratio will be 1:3. How many blue marbles were in the bag before I removed some?
B / Y =8/5,
[B -12] / [Y + 21] =1/3, solve for B, Y
B = 24 and Y = 15 - What you originally started with.