Question 363589
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Hi, 
Let x represent the amount of pure acid added (note: that is an 100% solution)
.60*5gal + 1.00*x = .80(5gal + x)
solve for x
.20x = .20*5gal
x = 5 gal of pure acid needs to be added.

checking our answer
.60*5gal + 5gal =  3gal + 5gal = .80*10gal = 8 gal