You cannot. You must know, or be able to derive, the length.
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If you do not know the length and width of ALL sides, then as far as I know it isn't possible to find the perimeter unless you measure each side and then find it.
To find the ratio of the length of a shape to its perimeter, you would divide the length by the perimeter. For example, if the length of a rectangle is 4 units and its perimeter is 12 units, the ratio would be 4/12 or 1/3. This ratio represents the proportion of the length to the total distance around the shape.
perimeter = 2 x (length + width) ⇒ length + width = perimeter ÷ 2 ⇒ length = (perimeter ÷ 2) - width So to find the length of the given rectangle, subtract the width from half the perimeter is how to do it. IF you meant "What is the length..." then: length = perimeter ÷ 2 - width = 40 ÷ 2 - 2 in = 20 - 2 in = 18 in
Length + length + width + width = perimeter. Length + width x 2 = perimeter
Perimeter = 2 * width + 2 * length, so rearranging --> width = (Perimeter / 2) - length