By solving a linear equation, we will see that we must use 12 gallons of 20% salt solution.
How many gallons of 20% we should use?
If we mix x gallons of 20% salt with 4 gallons of 40% salt, we will have a total mass of (x + 4 gall).
And the concentration on the left side is the known one, and we want the concentration of the mixture to e 25%, so we will have that:
x*0.20 + 4gal*0.40 = (x + 4gal)*0.25
(the percentages must be written in decimal form).
Now we can solve this linear equation for x:
x*0.20 + 4gal*0.40 = (x + 4gal)*0.25
4gal*0.40 = x*0.25 - x*0.20 + 4gal*0.25
4gal*0.40 - 4gal*0.25 = x*0.25 - x*0.20
4gal*0.15 = x*0.05
(4gal*0.15)/0.05 = x = 12
So you should use 12 gallons of the 20% salt solution.
If you want to learn more about linear equations, you can read:
https://brainly.com/question/14323743
