Step-by-step explanation:
it is not clear - is 500 miles the whole round trip, or is it only the trip in one direction ?
and do we need the time for the whole round trip, or only for the return trip ?
in any case, always remember,
speed = distance/time
therefore, to get time, we need to divide distance by speed :
distance / distance/time = time×distance/distance = time
so, let's calculate all these different scenarios :
1. 500 miles is only one direction, and we measure only the time going back with 60 mph :
500 / 60 = 50/6 = 25/3 = 8 1/3 = 8 hours 20 minutes.
as 1/3 of 1 hour = 1/3 of 60 minutes = 60/3 = 20 minutes.
2. 500 miles is only one direction (going 1000 miles altogether), and we measure the time going to with 50 mph and going back with 60 mph :
500 / 50 = 10 hours
500 / 60 = 8 hours 20 minutes
the whole round trip takes 18 hours 20 minutes.
3. 500 miles is the whole round trip. so, 250 miles is one direction. and we measure only the time going back with 60 mph :
250 / 60 = 25/6 = 4 1/6 = 4 hours 10 minutes.
as 1/6 of 1 hour = 1/6 of 60 minutes = 60/6 = 10 minutes.
4. 500 miles is the whole round trip. so, 250 miles is one direction. and we measure the time going to with 50 mph and back with 60 mph :
250 / 50 = 5 hours
250 / 60 = 4 hours 10 minutes
the whole round trip takes 9 hours 10 minutes.
did you notice ?
when going only half the distance, also the time it takes to travel that distance is only half (or vice versa - double). because the ratio distance/time is always the same.
is left, that means 
was spent. So 