A store is designing the space for rows of nested shopping carts. Each row has a starting cart that is 4 feet long, followed by the nested carts (so 0 nested carts means there's just the starting cart). The store measured a row of 13 nested carts to be 23.5 feet long, and a row of 18 nested carts to be 31 feet long. How much does each nested cart add to the length of the row?

Each nested cart adds feet to the length of the row.

If the store design allows for 43 feet for each row, how many total carts fit in a row?

total carts fit in a row.

Explain your reasoning for the total number of carts the store can fit in each row.