Let L, W and P denote length, width and perimeter.
L = (2/3)W - 5
The perimeter, P, is 2W + 2L, and this, in turn, is
2W + 2((2/3)W - 5), or 2W + (4/3)W - 10, or
(2 + 4/3)W - 10, or (10/3)W - 10
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