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How many gallons of hydrochloric acid must be added to 12 gallons of a 30% solution to obtain a 40% solution?
x be the extra added solution
30% of 12 gallons = 3.6 gall.
so forming the equation :
( 3.6 + x)/(12+x) = 40 / 100
solve for x you will get the required quantity which is 2 gallons