You have the misfortune to own an unreliable clock.
This one gains exactly 15 minutes every hour.
It is now showing 3.00am 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 15 minutes every hour, for every real hour that has passed, the clock will show 75 minutes.
Since the clock shows 3.00am, we know that 180 clock minutes have passed.
This equals 144 real minutes (180 / 75 x 60), so the clock shows 2.24 am.
The clock stopped 240 minutes ago (4 hours), so the time must now be 6.24am. QED.