We calculate oxidation number based on the rules of oxidation. For the unknown oxidation number we assign variable x, and for all the known values for alkali metals, oxygen and halogens, we use the fact that the algebraic sum of all the oxidation states in the compound is equal to the charge on the compound. The oxidation number for hydrogen is +1, Sodium (Na) is +1 and for oxygen is -2.

HPO^2-₃ è x= +3

Na₂S₄O₆ è x= 5/2

Na₂SO₃ è x= +4

Na₂SO₄ è x= +6

The fractional ON of 5/2 can be explained by its structure, where it contains S-S bond, thus the charge being divided by two atoms.