The length of a rectangle is two times the width and the perimeter is 90. find the dimensions of the rectangle.

Set the following: Length = L    Width = W

The length is twice the width:    L = 2*W     or:  W = L/2

Perimeter: P = 60ft = 2*L + 2*W

Substituting L for W in the perimeter equation gives:

P = 60ft = 2*L + 2*(L/2) = 2*L + L = 3L

L = 60ft/3 = 20ft    W=L/2=20ft/2=10ft

Verify result:  Perimeter= P = 60ft = 2L + 2W = 2*20ft +2*10ft = 40ft+20ft = 60ft

The area of the rectangular is: Area = A = Length*Width = L*W = 20ft*10ft = 200 (ft)^2