How many acres are in a square mile?

A square mile is a measure of area that is equivalent to a square whose sides each measure one mile in length. To find out how many acres are in a square mile, you need to know that one mile is equal to 5,280 feet. Therefore, when you calculate the area of a square mile in square feet, you multiply the length by the width, which is 5,280 feet by 5,280 feet. This results in 27,878,400 square feet in a square mile.

Since one acre is defined as 43,560 square feet, you can determine the number of acres in a square mile by dividing the total square feet in a square mile by the square feet per acre:

27,878,400 square feet / 43,560 square feet per acre = 640 acres.

Thus, there are 640 acres in a square mile, making this the correct answer.