<h3>Answer: width is 10 feet</h3>
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Work Shown:
L = length = 18
W = width = unknown
P = perimeter of rectangle = 56
P = L+L+W+W
P = 2L+2W
P = 2*18+2W ... plug in given length
P = 36+2W
36+2W = P
36+2W = 56 ... plug in given perimeter
36+2W-36 = 56-36 ... subtract 36 from both sides
2W = 20
2W/2 = 20/2 ... divide both sides by 2
W = 10
This rectangle is 10 ft by 18 ft.
As a check,
L+L+W+W = 18+18+10+10 = 36+20 = 56
so we get the proper perimeter
note: this assumes that there is no bit of fencing that goes to waste, there are no gaps, or there are no overlapping portions.
