Required Mathematical Intelligence

Chapter 100 The Two Watches Problem
I started two watches at the same time and later found out that one was 2 minutes behind an hour and the other was 1 minute an hour faster.When I went to look at the watch again, I found that the fast watch was an hour ahead of the slow watch.Ask: How much time has the watch passed?

[Answer: One watch is 3 minutes ahead of the other every hour, so after 20 hours, their time difference is 1 hour. ]
(End of this chapter)