What is the oxidation state of $S$ in $SO_4^{2-}$?

The oxidation state of sulphur in $H_2S_2O_7$ is

What is the order of the oxidation number of $S$ in $SO_3^{2-}$,$S_2O_4^{2-}$,and $S_2O_6^{2-}$?

What is the oxidation number of carbon in $CH_2O$?

$A$ compound contains atoms of three elements $A$,$B$ and $C$. If the oxidation number of $A$ is $+2$,$B$ is $+5$ and that of $C$ is $-2$,the possible formula of the compound is:

What is the change in oxidation number of $Cr$ in the following redox reaction?
$3H_2O_2(aq) + Cr_2O_7^{2-}(aq) + 8H^+(aq) \rightarrow 3O_2(g) + 2Cr^{3+}(aq) + 7H_2O(l)$