Solving a chemistry problem

3. When determining Mn of steel, a 0.200 g sample of it was dissolved, Mn was oxidized to MnO4- and the solution was diluted to 0.200 ml. Photometry was carried out by a differential comparison method with a standard solution Cst = 20 mg/l and a zero solution with a KMnO4 concentration of 0.0005 N. The optical densities of the test solution and the standard solution relative to the zero solution were Ast = 0.28; Ax=0.15.