20. Solving a chemistry problem

20. The determination of nickel in steel was carried out by the photometric method with dimethylglyoxime. A sample of steel 0.2010 g after dissolution was transferred into a volumetric flask with a capacity of 100.0 ml. For photometry, 5.0 ml of the resulting solution was placed in a volumetric flask with a capacity of 200.0 ml. The optical density of this solution was 0.300. In another volumetric flask with a capacity of 200.0 ml, a standard solution containing 0.045 mg of nickel was placed and prepared for photometry in the same way as the test solution. The optical density of the standard solution was 0.330. Calculate the mass fraction of nickel in the test steel.