Hi, Let x repesent the amount of acid added .50*5gal +1.00x = .80(5+x) Simplify and Solve .50*5gal +1.00x = .80*5+ .8x .2x = .80*5 - .50*5 .2x =.3*5 .2x = 1.5 x = 7.5gal checking our answer 2.5gal + 7.5gal = .8* 12.5gal = 10gal