i'll just assume you meant he has to walk 24 miles to school since 12 + 12 isn't 1. if he normally walks 3 miles an hour for 24 miles, his average speed is also 3 miles an hour. if he walks 2 miles an hour for the first half of his 24 mile journey, he has to walk 4 miles an hour for the second part, since he has to reach his average speed of 3 miles an hour