111. Solving a chemistry problem

4.2 A portion of the preparation containing vitamin B12, weighing 0.4230 g, was dissolved in a 50 ml flask. Aliquots of 20 ml each were taken into two volumetric flasks of 50 ml each. A solution containing 0.0134 g of vitamin B12 was added to one of them. The solutions in the flasks were diluted with water to the mark. The optical densities of these two solutions, measured under identical conditions, turned out to be 0.23 and 0.54. Calculate the mass fraction of vitamin B12 in the preparation.