Answer: The average speed on the afternoon route is 40 miles per hour.
Explanation:
Let the average speed of afternoon route be 'x'.
Let the average speed of morning route be 'x-12'.
Distance covered on the morning route = 70 miles
Distance covered on the evening route = 100 miles
Since they take the same amount of time.
So, our equation becomes,

Hence, the average speed on the afternoon route is 40 miles per hour.