Answer
Because $1~mi^2= 640~acres$, the unit fraction needed to convert from acres to square miles is $\frac{1~mi^2}{640~acres}$ and the unit fraction needed to convert from square miles to acres is $\frac{640~acres}{1~mi^2}$
Work Step by Step
Because $1~mi^2= 640~acres$, the unit fraction needed to convert from acres to square miles is $\frac{1~mi^2}{640~acres}$ and the unit fraction needed to convert from square miles to acres is $\frac{640~acres}{1~mi^2}$
