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.