Edward C. answered 02/03/15
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Caltech Grad for math tutoring: Algebra through Calculus
Let W = width, L = length.
We are given that W = L/3, so 3W = L.
The perimeter is 56 = W + L + W + L.
Substituting 3W for L gives W + 3W + W + 3W = 56 ⇒ 8W = 56 ⇒ W = 7, so L = 21.
The dimensions of the rectangle are 7 feet by 21 feet.