You have the misfortune to own an unreliable clock. This one loses exactly 10 minutes every hour. It is now showing 1.00am and you know that is was correct at midnight, when you set it. The clock stopped 1 hour ago, what is the correct time now?
Answer: 2.12am: since the clock is losing 10 minutes every hour, for every real hour that has passed, the clock will only show 50 minutes. Since the clock shows 1:00am, we know that 60 clock minutes have passed. This therefore equals 72 real minutes and hence 1.2 hours. The clock stopped 1 hour ago and the time must now be 2.12am. QED.