79. Solving a chemistry problem

3. To determine sodium in glass, three 0.100 g samples were dissolved in a mixture of acids, the solutions were evaporated, the residues were treated with diluted HCl and transferred to 250.00 ml measuring flasks. To the second and third flasks were added, respectively, 10.00 and 20.00 ml of a standard sodium solution (c = 250.0 μg / ml). The solutions were diluted with water to the mark, sprayed in a flame atomizer and their optical densities were determined A1 = 17.0; A2 = 26.0; A3 = 35.0. Determine the mass fraction (in %) of sodium in the glass.