This problem is over-constrained.   In other words, the answer is easily determined by subtracting the starting 15L from the final 40L to 

get .  A better problem would not specify how many liters the final mixture has and would simply ask how many liters need to be added to bring the concentration down to 35%.






The problem would ordinarily be solved as follows:



 Let x = amount of 20% acid solution to be mixed in

 



 









 

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