$3 MnO _{4}^{2-}+3 H ^{+} \longrightarrow 2 MnO _{4}^{-}+ MnO _{2}+2 H _{2} O$
O.S. of $Mn$ in $MnO _{4}^{-}=+7$
O.S. of $Mn$ in $MnO _{2}=+4$
$\text { difference }=3$
[પરમાણુ ક્રમાંક $Eu, \,63;\, Sm ,\, 62 ;\, Tm ,\, 69 ;\, Tb ,\, 65 \text {; Yb, 70; Dy, 66] }$
$FeCr _{2} O _{4}+ Na _{2} CO _{3}+ O _{2} \rightarrow A + Fe _{2} O _{3}+ CO _{2}$
$A + H ^{+} \rightarrow B + H _{2} O + Na ^{+}$