Hi, Let x represent the amount of the 100% pure acid solution 1.00x + .60*4gal = .80*(x + 4gal) solving for x .20x = .20*4gal x = 4 gal of pure acid needed. CHECKING our Answer 4gal + 2.4gal = 6.4gal = .8*8gal = 6.4gal