Length = l
Width = w
Perimeter = p = 100
Perimeter of rectangle = 2(l+w)
100 = 2 (4w + w)
100 = 2(5w)
100 = 10w
100/10 = w
10 = w
w = 10
Area of rectangle = length * width
a = l*w
a = 4w*w
a = 4w^2............(1)
Put the value of w in (1)
a = 4(10)^2
a= 4(100)
a = 400 yd^2