MnO42− (manganate) has Mn in +6 oxidation state.
In acidic medium, it undergoes disproportionation where Mn is simultaneously oxidized and reduced:
Oxidation: +6→+7 (forms MnO4−, permanganate)
Reduction: +6→+4 (forms MnO2)
Reaction: 3MnO42−+4H+→2MnO4−+MnO2+2H2O