Question 964328
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The 40% acid solution is 60% water. 60% of 200cl
(0.60)(200cl)=120cl of water
A 50% solution would be 50% water:
120cl/0.50=240cl 
120cl of water will make 240cl of 50% solution, so 200cl of 40% solution (120cl water and 80cl acid) needs 40cl of acid added to make 240cl of 50% solution.