I really must get a new watch.
The one I have loses exactly 20 minutes every hour.
It now shows 4:00am and I know that it was correct at midnight, when I last set it.
I happen to know that the watch stopped 4 hours ago, so what is the correct time now?
Since the watch is losing 20 minutes every hour, for every real hour that has passed, the watch will only show 40 minutes. Since the watch shows 4.00am, we know that 240 watch minutes have passed. This therefore equals 360 real minutes and hence 6 hours. The watch stopped 240 minutes ago, therefore the time must now be 10.00am. QED.