Question 721557
The total liters of each of the 20% and 4% solution must add up to 40.

(0.2(x) + 0.04(40-x))/40 = 0.05

0.15(x) + 1.6 = 2

x = 0.4/0.15 = 2.67 L of 20% solution.

37.33 L of 4% solution.