SOLUTION: how many liters of 20% saline solution should be added to 40 liters of a 50% saline solution to make a 30% solution?

Question 618603: how many liters of 20% saline solution should be added to 40 liters of a 50% saline solution to make a 30% solution?
Answer by dragonwalker(73) (Show Source): You can put this solution on YOUR website!
You have 40 liters of 50% solution. You want a 30% solution and have a 20% solution to do this with.
50% is 20% more than the desired concentration
20% is 10% less than the desired concentration
so twice as much 20% solution is needed to lower the concentration compared to the 50% solution raising the concentration (10 vs. 20)
So if there is 40 liters of the 50% which is 20% above the desired concentration twice as much of the 20% is needed to lower it by the same amount.
So 40 x 2 liters of 20% is required. i.e. 80 liters

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