Question 97755
First I ask: how much acid is in the original solution?
It's {{{.4*2 = .8}}} gallons
Call the gallons of acid you want to add {{{x}}}
{{{(x + .8) / 2 = .7}}}
{{{x + .8 = 1.4}}}
{{{x = .6}}}
So, you add .6 gallons of acid to get a 70% solution