Arthur D. answered 10/02/14
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let x=# of gallons of water that must evaporate
the amount of acid doesn't change
you have 200 gallons of a 30% acid solution
you want (200-x) gallons of a 40% acid solution
30%(200)=40%(200-x)
0.3(200)=0.4(200-x)
60=80-0.4x
60-80=-0.4x
-20=-0.4x
20=0.4x
200=4x
200/4=x
50 gallons=x