You have the misfortune to own an unreliable clock. This one loses exactly 8 minutes every hour. It is now showing 4:15pm and you know that is was correct at midnight, when you set it. The clock stopped one hour ago, what is the correct time now?
Answer: 7:45pm: since the clock is losing 8 minutes every hour, for every real hour that has passed, the clock will only show 52 minutes. Since the clock shows 4.15pm, we know that 975 clock minutes have passed. This therefore equals 1125 real minutes and hence 18 hours 45 minutes. The clock stopped 60 minutes ago, therefore the time must now be 19.45pm. QED.