Answer
$2\displaystyle \frac{2}{3}$ gal
Work Step by Step
Set up a table:
$\left[\begin{array}{ccc}
\text{strength} & \text{gal of solution} & \text{gal of pure alcohol}\\
100\% & x & x\\
15\% & 20 & 3\\
& & \\
25\% & (x+20) & 0.25(x+20)
\end{array}\right]$
The equation to solve is:
$ x+3=0.25(x+20)\qquad$ ... multiply with $4$
$ 4x+12=x+20\qquad$ ... add $(-x-12)$
$3x=8$
$x=\displaystyle \frac{8}{3}=2\frac{2}{3}$ gal