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

12 gallons of 20% salt solution

Step-by-step explanation:

Let

x > the number of gallons of 20% salt solution

we know that

20%=20/100=0.20

40%=40/100=0.40

25%=25/100=0.25

so

0.20(x)+0.40(4)=0.25(x+4)

Solve for x

0.20x+1.60=0.25x+1

0.25x-0.20x=1.60-1

0.05x=0.60

x=12 gallons