Question 102385
Let x=amount of pure acid needed

Now we know that the amount of pure acid in the acid that's added (x) plus the amount of pure acid in the 40% solution ((7)*(0.40)) equals the amount of pure acid in the final mixture ((0.70)(7+x)).  So our equation to solve is:

x+7*0.40=0.70(7+x)  get rid of parens

x+2.8=4.9+0.70x  subtract 0.70x and also 2.8 from both sides

x+2.8-2.8-0.70x=4.9-2.8+0.70x-0.70x  collect like terms

0.30x= 2.1  divide both sides by 0.30

x=7 gallons---------------------------amount of pure acid needed

CK
7+7*0.40=0.70(14)

7+2.8=9.8
9.8=9.8

Hope this helps---ptaylor