145. Solving a chemistry problem

75. In the determination of zinc using standard additives, four volumetric flasks with a capacity of 50.00 ml were taken. In each of them they poured 20.00 ml of the solution being analyzed and in the second, third and fourth 2.00; 4.00 and 6.00 ml of a standard solution with a concentration of 0.50 μg / ml. The volumes of solutions in volumetric flasks brought to the mark with distilled water. The atomic absorption of these solutions at 213.9 nm is 12, 16, 20, and 24 units, respectively.
Determine the concentration (mg / l) of zinc in the analyzed solution graphically.