The answer is that the circumference of each small rectangle is 20*2/4= 10. Because the perimeter of a big rectangle is composed of the length and width of each small rectangle, and the perimeter of each small rectangle is (length+width) *2, so the perimeter of a big rectangle multiplied by 2 is the sum of the perimeters of four small rectangles, and then divided by 4 is the perimeter of each small rectangle.