Question

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James and John had set their pocket watches to the 
same time. Unknown to them, John's watch was running 
exactly two minutes per hour slow, and the watch 
belonging to James was going exactly a minute per hour 
too fast. Later, when they checked their watches again, it 
was discovered that the watch belonging to James was 
exactly one hour ahead of John's watch. 
How long had it been since they had originally set 
their watches?

Answer 1

This case the difference btw John's  and James watches is 3 min per hour. So it had been 60 min/3=20 hours  since they had set their watches