Final answer:
To determine how many horses each Pony Express rider used, divide the total miles each rider covered (156 miles) by the miles ridden per horse (13 miles), resulting in 12 horses used per rider.
Step-by-step explanation:
The question involves calculating how many horses each Pony Express rider used if they traveled an equal distance of a 312-mile trip and changed horses every 13 miles. To find the answer, we divide the total distance each rider traveled by the distance covered per horse.
First, we split the total trip distance between the two riders: 312 miles ÷ 2 riders = 156 miles per rider. Next, we calculate the number of horse changes per rider by dividing the distance each rider traveled by the distance covered before changing horses: 156 miles ÷ 13 miles per horse = 12 horses per rider.
Therefore, each Pony Express rider used 12 horses to cover their part of the trip.