What is the oxidation number of underlined species in $\underline{P}F_6^{-}$ and $\underline{V}_2 O_7^{4-}$ ions respectively?

The oxidation number of $C$ in $CO_2$ is

What is the oxidation state of oxygen in $OF_2$?

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

What is the oxidation number of $Cl$ in $HClO_4$ and $HClO_3$?

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