Average oxidation number of iodine in ${\mathrm{KI}}_3$

• Let us assume $\mathrm{X}$ be the oxidation state of iodine in ${\mathrm{KI}}_3$. Potassium has one  

Electron in its valence shell, so it exists in +1 oxidation state.

           So, $1+3X=0$

              $3 X=-1$

                        $\mathrm{X}=\frac{-1}{3}$