The difference between the area of the big and small squares is 40 square meters, and the sum of the side lengths is 20 meters
Side length difference: 40 △ 20 = 2 (CM)
Side length: (20 + 2) △ 2 = 11 (CM)
(20-2) △ 2 = 9 (CM)
Area: 11 × 11 = 121 (square centimeter)
9 × 9 = 81 (square centimeter)