145. Solving a chemistry problem

74. When determining magnesium using the “standard addition method”, 10.00 ml of the analyzed solution were poured into two volumetric flasks with a capacity of 25.00 ml and 10.00 ml of a standard magnesium solution with a concentration of 1.00 μg / ml were added to one of the flasks. After that, the volumes of solutions in volumetric flasks were brought to the mark with distilled water.
Atomic absorption of these solutions at 285.1 nm is equal to 20 and 38 units, respectively.
Determine the concentration (mg / l) of magnesium in the analyzed solution.