Perimeter = 2 (length + width)
To get a perimeter of a rectangle of 20 units, you need the length + width as 10 units. The combination possibilities would be 1and 9, 2 and 8, 3 and 7, 4 and 6. Use any one set and find the area
Area = length * width
area= 1*9 = 9 units^2
area = 2*8 = 16 units^2
area = 3*7 = 21 units^2
area = 4*6 = 24 units^2