Question 576777
How many gallons of hydrochloric acid must be added to 12 gallons of a 30% solution to obtain a 40% solution?
sol:
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