Watch it !
I started two watches at the same time. It turned out that one of them went two minutes per hour too slow, and the other went one minute per hour too fast. When I looked at them again, the faster one was exactly one hour ahead of the other. How long had the watches been running?
Answer: The faster watch gains on the slower one at the rate of three minutes every hour. Thus, after 20 hours, the faster on will be ahead by one hour.