Determine the oxidation state of $Cr$ in $K_2Cr_2O_7$.

The oxidation number of sulphur in $H_2S$ is

Oxidation number of $Cl$ in $CaOCl_2$ (bleaching powder) is

Assign the oxidation number to the underlined element in each of the following species:
$1$. $K_2\underline{Mn}O_4$
$2$. $Ca\underline{O}_2$
$3$. $Na\underline{B}H_4$
$4$. $KAl(\underline{S}O_4)_2 \cdot 12H_2O$

Oxidation numbers of $P$ in $PO_4^{3-}$,of $S$ in $SO_4^{2-}$ and that of $Cr$ in $Cr_2O_7^{2-}$ are respectively

What is the oxidation number of sulphur in $Na_2S_4O_6$?