Given below are half-cell reaction:
`Mn^(2+)+2e^(-) rarr Mn,, E^(@) = -1.81 V`
`2(Mn^(3+)+e^(-) rarr Mn^(2+)),, E^(@) = +1.51 V`
The `E^(@)` for `3Mn^(2+) rarr Mn+2Mn^(3+)` will be: