A rectangle has a width of 10 feet and a length of 40 feet. What is the length of the side of a square with the same area?

First find the area of the rectangle: 10 * 40 = 400

Now, since the square has the same area, we'll find the side lengths of the square by just finding the square root of 400 which is 20. You can do this with a calculator. So the side length of any side on the square is 20.