What is the oxidation number of $Cr$ in $K_{2}Cr_{2}O_{7}$?

Calculate the oxidation number of $Cr$ in $CrO_4^{2-}$ ion and $K_2Cr_2O_7$ respectively.

Given below are two statements.
Statement $I$ : In $H_2O_2$, each oxygen atom is assigned an oxidation number of $-1$, and in $RbO_2$, each oxygen atom is assigned an oxidation number of $-\frac{1}{2}$.
Statement $II$ : The representation of $HAuCl_4$ and $MnO_2$ in Stock notation is $HAu(III)Cl_4$ and $Mn(IV)O_2$ respectively.
Examine the above statements and choose the correct answer.

The oxidation number of $N$ in $N_3H$ is ..........

What is the oxidation number of $Fe$ in $Fe_3O_4$?

What is the change in oxidation number of selenium in the following redox reaction?
$SeO_{3(aq)}^{2-} + Cl_{2(g)} + 2OH^- \longrightarrow SeO_{4(aq)}^{2-} + 2Cl_{(aq)}^- + H_2O$