Neither one. If you insist on finding the length of the perimeter using a proportion, you might try
(1 mi)/(5280 ft) = (2*3/8 mi)/(x ft)
You will find that 3/4 of a mile of fence is 3960 ft.
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In a proportion, similar units are found in corresponding places.
(1 mi)/(5280 ft) = (2*3/8 mi)/(x ft)
You will find that 3/4 of a mile of fence is 3960 ft.
_____
In a proportion, similar units are found in corresponding places.