51. Solving a chemistry problem

To determine sodium in glass, three portions of sodium 0.1000 g each were dissolved in a mixture of H2SO4 and HF, the solutions were evaporated, the residues were treated with dilute HCl and transferred into volumetric flasks with a volume of 250.0 ml. 10.0 and 20.0 ml of standard sodium solution (c = 250 µg/ml) were added to the second and third flasks, respectively. The solutions were diluted with water to the mark and the light gas, air, was photometered in a flame. The following results are obtained:
Solution 1 2 3
h, c.u. 17.0 26.0 35.0
Determine the mass fraction (%) of sodium in the glass.