A- y= -3/2x+8
B- y=3/2x+5
C- y=2/3x+8
D- y=-2/3x+5

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