SOLUTION: A hospital needs 40 L of a solution of disinfectant. How many liters of a 20% solution and a 4% solution should be mixed to obtain this 15% solution?

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.