Question 944223
20 L is 40% of (20 L)/0.40= 50 Liters, so total volume of a 40 % solution with 20 L of acid would be 50 L.  Presently we have 20 L acid + 50 L water= 70 L of solution, we need to evaporate 20 L to reach 50 L and 40% concentration.