SOLUTION: how many ml of pure alcohol and how many ml of 4% alcohol solution must be combined to make up 480 ml of an 8% alcohol solution?

 We have    x    mL of the 8% solution, and

       (480 - x) mL of the pure alcohol. 



x mL of the 8% solution contribute 0.08x mL of the pure alcohol to the new mixture

(480-x) ml of the pure alcohol contribute (480-x) mL of the pure alcohol to the new mixture.



Now we write the balance equation for the pure alcohol

    0.04x + (480-x) = 0.08*480  milliliters


saying that the amount of the pure alcohol in ingredients (left side)
is the same as the amount of the pure alcohol in the final 8% solution (right side).



From the equation

    x =  = 460.



ANSWER.  460 mL of the 4% solution, and the rest, 480 mL - 460 mL = 20 mL of the pure alcohol should be mixed.