I have a bag with yellow and blue marbles in it. At the moment, the ratio of blue marbles to yellow marbles is 4:3. If I add 5 blue marbles and remove 3 yellow marbles, the ratio will be 7:3. How many blue marbles were in the bag before I added more?
You can construct two equations from this as follows:
Let b be the original number of blue marbles and y the original number of yellow marbles.
Solve these simutaneous equations for b.