Average speed for the entire trip, both ways, is
(Total distance) divided by (total time) .
We don't know the distance from his house to the gift store,
and we don't know how long it took him to get back.
We'll need to calculate these.
-- On the trip TO the store, it took him 50 minutes, at 6 mph.
-- 50 minutes is 5/6 of an hour.
-- Traveling at 6 mph for 5/6 of an hour, he covered 5 miles.
-- The gift store is 5 miles from his house.
-- The total trip both ways was 10 miles.
-- On the way BACK home from the store, he moved at 12 mph.
-- Going 5 miles at 12 mph, it takes (5/12 hour) = 25 minutes.
Now we have everything we need.
Distance:
Going: 5 miles
Returning: 5 miles
Total 10 miles
Time:
Going: 50 minutes
Returning: 25 minutes
Total: 75 minutes = 1.25 hours
Average speed for the whole trip =
(total distance) / (total time)
= (10 miles) / (1.25 hours)
= (10 / 1.25) miles/hours
= 8 miles per hour