Question

The equation a = 640s gives the relationship between s square miles and a acres. Bob owns 3.5 square miles of farmland. How many acres does he own?

Answer 1

acres = 640 * sq miles
acres = 640 * 3.5
acres = 2,240 square miles

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Incidentally, here's how that 640 acres per square mile is derived.
1 acre = 43,560 square feet
1 square mile = 5,280^2 = 27,878,400 square feet
27,878,400 / 43,560 = 640 acres exactly