The perimeter of the rectangle is 40dm, and the width is 4cm less than the length. How many square decimeters

The perimeter of the rectangle is 40dm, and the width is 4cm less than the length. How many square decimeters is the area of a rectangle less than the area of a square with the same perimeter?

Given:
Ppr = 40dm
L – the length of the rectangle
H = L-0.4 – rectangle width
Sq = (Ppr / 4) ^ 2 = 100 dm2
Spr = L * (L-0,4)
Ppr = 2 * L + 2 * (L-0.4) = 4L-0.8
40 = 4L-0.8
4L = 40.8
L = 10.2 dm
Spr = L * (L-0.4) = 10.2 * (10.2-0.4) = 10.2 * 9.8 = 99.96 dm2
Sq-Spr = 100 dm2-99.96 dm2 = 0.04 dm2 – the difference between the required areas