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.