2. Solving a chemistry problem

5. 6.00 and 9.00 cm3 of a standard solution of Fe3 salt with a titer of 0.0100 mg/ml were placed in volumetric flasks with a capacity of 25.00 cm3. 20.00 cm3 of analyzed water was introduced into the third flask. After adding a photometric reagent and diluting the solutions with water to the mark, the optical densities of the solutions are respectively equal to 0.33; 0.68 and 0.51. Calculate the mass concentration of Fe3 in the analyzed water.