Question 128656
Let x=amount of 15% solution needed

Total solution after the mixture takes place is 12+x gal, right?

Now we know that the amount of pure salt in the 15% solution(0.15x) plus the amount of pure salt in the 12 gal (0.25*12) has to equal the amount of pure salt in the final mixture (0.21*(12+x)).  So our equation to solve is:

0.15x+0.25*12=0.21(12+x)   get rid of parens and simplify

0.15x+3=2.52+0.21x  subtract 0.15x and also 2.52 from both sides

0.15x-0.15x+3-2.52=2.52-2.52+0.21x-0.15x  collect like terms

0.48=0.06x  divide both sides by 0.06

x=8 gal--------------- amount of 15% solution needed

CK

0.15*8+0.25*12=0.21*20
1.2+3=4.2
4.2=4.2


Hope this helps----ptaylor