It's not necessary to know the distance or time taken, we know the time going and the time returning, distance is the same either way
10mph and 12mph, or 22 mp 2h
Divide by 2 hours = 11mph
(Note is 'per hour' not 'in an hour', only 10 miles are run back, in less than an hour - but the actual time and distance are still irrelevant)
10 mph is the same as 10 mp 60mins
12 mph is the same as 12 mp 60mins
1 mp 6 mins, 1 mp 5 mins
2 mp 11 mins = 10 mp 55 mins
We need to add 5 minutes for 'per hour'
1 mile would be 5.5 mins (or 5:30)
Shocker - it's not 11mph but 10 point something