We need 2 equations to solve for the 2 unknowns.
Let V1 = volume of 5% solution and let V2 = volume of 40% solution
The equations are as follows:
V1 + V2 = 10
(0.05 * V1) + (0.4 * V2) = 10 * 0.2 = 2
Let's solve by substituting one equation into the other:
V1 = 10 - V2
0.05(10 - V2) +0.4V2 = 2
0.5 - 0.05V2 +0.4V2 = 2
0.35V2 = 1.5
V2 = 4.3 liters
V1 = 10 - 4.3 = 5.7 liters