Question 380612
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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