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.