If [tex]x[/tex] is the amount (gal) of the 20% solution to be used, then the mixture would have a total volume of [tex]x+4[/tex] gal. We want to end up with a 25% salt solution, so that
[tex]0.2x+0.4\cdot4=0.25(x+4)[/tex]
[tex]\implies0.2x+1.6=0.25x+1[/tex]
[tex]\implies0.6=0.05x[/tex]
[tex]\implies x=12[/tex]
