Consider that the area (A) and perimeter (P) of a rectangle with length (L) and width (W) is given by the formulae,

Given that the perimeter of the rectangular garden 240 ft,

Also, it is given that the length is twice the width,

Substitute this in equation (1) and simplify,

Then the corresponding length will be,

Now that the length and width of the rectangular garden is known, the area can be calculated as,

Thus, the length, width, and area of the rectangular garden are 80 feet, 40 feet, and 3200 square feet, respectively.