Given that XY = ZY, WX = 6x − 1, and XZ = 10x + 16, find ZW.

We have 12 liters of 8% acid solution to which we add an unknown amount of liters (x) of 20% acid solution to make a 16% acid solution.
.08 * 12 +.20x = .16 * (12 +x)
.96 + .20x = 1.92 + .16x
.04x = .96
x = 24 liters of 20% acid solution must be added

Source http://www.1728.org/mixture.htm
see example B