Answer:
The answer is 19.5 miles per hour.
Explanation:
If it takes Louisto reach the store 15 minutes at 6 miles per hour, the distance between the store and his home is 1.5 miles.
His speed travelling back is 20 miles per hour, which means that the 1.5 mile trip is completed in 4.5 minutes.
The total time he travels is 19.5 minutes and the total distance is 3 miles, so his average speed is Vavg = (3 * 60) / 19.5 = 9.23 miles per hour.
I hope this answer helps.