You have the misfortune to own an unreliable clock. This one loses exactly 20 minutes every hour. It is now showing 8.00am and you know that is was correct at midnight, when you set it. The clock stopped 10 hours ago, what is the correct time now?
Since the clock is losing 20 minutes every hour, for every real hour that has passed, the clock will only show 40 minutes. Since the clock shows 8:00am, we know that 480 clock minutes have passed. This therefore equals 720 real minutes and hence 12 hours. The clock stopped 10 hours ago and so the time must now be 10.00pm. QED.