the answers comes to be for every 1 unit of pure acid (100%) add three units of 20% and you get the desired 40%
so if you need 40 L (i don't know what container sizes there are so i can't give you an exact amount just add 1 part 100% acid and 3 parts 20% acid)
To make a 40-liter batch of 40% acid solution, combine 10 liters of pure acid (100%) with 30 liters of 20% acid solution. This results in a total of 40 liters with the required concentration. This mixing achieves the desired acidity for the batch.
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