Question 386566
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Hi,         
Let x and (40L-x) represent the 20% and 4% solutions respectively
Mixing the 20% and 1%  to form 40L of 15% solution
 .20x + .04(40L-x) = .15*40L
              .16x = .11*40L
                 x = .11*40L/.16
                 x = 27.5L, the amount of the 20% solution.  
                     12.5L,the amount of the 4% solution  (40L - 27.5L)
CHECKING our Answer
 .20*27.5L + .04*12.5L = 5.5L + .5L = 6L = .15*40L
Which tells us incidentally there is 6L of disinfectant in the final solution.