The formula for the perimeter of a rectangle is Perimeter = 2(Length) + 2(Width).
The width is x, the length is 3x, and the perimeter is 40.
40 = 2(3x) + 2(x) >> 40 = 6x + 2x >> 40/8 = 8x/8 >> x = 5
Width = x = 5 feet Length = 3x = 3(5) = 15 feet
To check substitute into the original formula:
40 = 2(3(5)) +2(5) = 2(15) +10 = 30 + 10 = 40
So, the dimensions are 5 ft by 15 ft.