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You have the misfortune to own an unreliable clock. This one gains exactly 12 minutes every hour.

It is now showing 10pm and you know that is was correct at midnight, when you set it.

The clock stopped four hours ago, what is the correct time now?

Since the clock is gaining 12 minutes every hour, for every real hour that has passed, the clock will show 72 minutes.

Since the clock shows 10.00pm, we know that 22 x 60 = 1320 clock minutes have passed. 1320 ÷ 72 x 60 = 1100.

This therefore equals 1100 real minutes and hence 18 hours 20 minutes = 6:20pm.

The clock stopped 4 hours ago, therefore the time must now be 10.20pm. QED.

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