The oxidation number of S in ${\mathrm{H}}_2{\mathrm{S}}_2{\mathrm{O}}_8$ is

The chemical structure of   ${\mathrm{H}}_2{\mathrm{S}}_2{\mathrm{O}}_8$ is as follows:

So the oxidation number of S should be :

2 x (+ 1) + 2 x X + 6 x (- 2)  + 2$\times$( -1)= 0 or X = + 6

(for H)        (for S)    (for O) (for O - O)