Question

How many gallons of 20% salt solution must be mixed with 4 gallons of 40% salt solution to make 25% salt solution?

Answer 1

If $x$ is the amount (gal) of the 20% solution to be used, then the mixture would have a total volume of $x+4$ gal. We want to end up with a 25% salt solution, so that

$0.2x+0.4\cdot4=0.25(x+4)$

$\implies0.2x+1.6=0.25x+1$

$\implies0.6=0.05x$

$\implies x=12$