Question 618603
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