Consider the question. Johnny has been playing guitar for a while now. He practices the same amount each day. As of 20 days ago he had half as much practice as he has currently. How many days will pass before Johnny has 3 times as much practice as he does currently?
If the answer is '80'; explain the reasoning:
If Johnny had half as much practice as he currently has as of 20 days ago, this means he has 20 * 2 = 40 days worth of practice currently. Therefore, to have three times as much practice as he currently has, he'd need to practice for 40 * 3 = 120 days. Since he's already practiced for 40 days, this means he needs to practice or 120 - 40 = 80 more days.