Question 335003
You get two equations, one based on total volume, and one based on the mix:

x + 10 = y

x(0.4) + 10(0.8) = y(0.6)

Substitute the first equation for "y" into the second equation and solve for "x":

x(0.4) + 10(0.8) = (x + 10)(0.6)

x(0.4) + 8 = x(0.6) + 6

x(0.4) - x(0.6) = 6 - 8

x(-0.2) = -2

x = -2/-0.2 = 10

Therefore, y = 20.

10 liters of a 40% alcohol solution must be mixed with 10 liters of a 80% alcohol solution to get a 20 liters of a 60% alcohol solution.