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If you'll use x liters of a 70% solution, then for make 50 liters of solution you have to add (50-x) liters of a 20% solution.
In x liters 70% solution is 0.7x liters substance, in (50-x) liters 20 % solution is
substance, we will make 50 liters 60% solution, so will have
liters of substance
liters of a 70% solution, and (50-40)=10 liters of a 20% solution