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I was recently given a new watch for my birthday.

However, as usual with my presents, it was quite useless as it loses 6 minutes every hour.

I set it using my friend's accurate watch at midnight and it now shows 10.39am.

I know that the watch stopped 21 minutes ago so what is the correct time now?

Since the watch is losing 6 minutes every hour, for every real hour that has passed, the watch will only show 54 minutes (i.e. for every 54 clock minutes, 60 real minutes have passed)

Since the watch shows 10:39am we know:

10:39am = (10 x 60) + 39 = 639 watch minutes

for every 54 watch minutes, 60 real minutes have passed:

(639 ÷ 54) x 60 = 710 real minutes

and converting this to hours:

710 minutes = 11 hours 50 minutes

the watch stopped 21 minutes ago, so the current time is:

12.11pm. QED.

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