Answer:
7.5
Explanation:
The relation between time, speed, and distance is ...
time = distance/speed
If distance is "1 round trip", then the time going is ...
going = 0.5/(10 mi/h) . . . . for 1/2 round trip
and the time coming is ...
coming = 0.5/(6 mi/h)
Then the average speed for the full round trip is ...
speed = distance/time
average speed = 1/(going + coming) = 1/(0.5/10 +0.5/6) mi/h
= 1/((3+5)/60) mi/h
= 60/8 mi/h = 7.5 mi/h
Jack's average speed for the round trip was 7.5 mph.