How many gallons of a 30% salt solution must be added to 40 gallons of a 12% salt solution to obtain a 20% solution?

Let’s call the number of gallons of 30% solution "x". If we mix together 40 gallons and x gallons of solution, we’re going to come out with a total of 40 + x gallons of that 20% solution. Mathematically, this is

40 (12%) + x (30%) = (40 + x) (20%)

With the percents as decimals:

40 (0.12) + x (0.3) = (40 + x) (0.2)

Simplifying, we have:

4.8 + 0.3x = 8 + 0.2x

Collecting the x terms on one side and the constants on the other:

0.1x = 3.2

And multiplying both sides by 10 to find x:

x = 32

So, you’ll need 32 gallons of 30% solution to yield 40 + 32 = 72 gallons of 20% solution.