$3 \overset{+6}{\mathrm{MnO}_{4}^{-2}}+4 \mathrm{H}^{+} \rightarrow \overset{+4}{\mathrm{MnO}_{2}}+\overset{+7}{\mathrm{MnO}_{4}^{-}}$ 

$\mathrm{Mn} \rightarrow 4 s^{2} 3 d^{5}$ 

$\mathrm{Mn}^{+4} \rightarrow 3 d^{3}$ 

$\mathrm{n}=3$ 

$\begin{aligned} \mu &=\sqrt{n(n+2)} \\\\ &=\sqrt{3(5)} \\\\ &=\sqrt{15} \\\\ &=3.87 \approx 4 \text { B.M. } \end{aligned}$