A 60% solution is made up of the sum from 20 units of an 80% solution and X units of a 40% solution.
.60 = (20 * .80 + X * .40) / (20 + X)
.60 * (20 + X) = 20 * .80 + X * .40
.60 * (20 + X) = 20 * .80 + X * .40
12 + .6X = 16 + .4X
.2X = 4
X = 20
You'll need 20 liters.