Question 381637
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Hi,         
Let x represent the amount of the 10% Solution, 
then (20L +x) will be the amount of the final solution
Mixing to form the solution***
 .10x + .40*20L = .20(x + 20L)
Solving for x
  .20*20L = .10x
  .20*20L/.10 = x
      40L = x, the amount of the 10% Solution

CHECKING our Answer***
 .10*40 + .40*20 =  4L + 8L = .20*60 = 12