If Billy needs 4000 points then it tells us how much is needed. Then it tells us he has compleated 3/5 of it. In order to get how much he still has left, you just multiply the two numbers together and then subsract that number from 4000.
4000 * 3/5 = 2400 (That is how much he has completed)
4000 - 2400 = 1600
Billy still needs 1600 points.
