In 2MnO4-, the oxidation number of Mn can be calculated by considering the overall charge of the compound and the oxidation numbers of the other atoms.

The overall charge of 2MnO4- is -2. Since there are two oxygen atoms in each MnO4- ion, and each oxygen atom has an oxidation number of -2, the total charge contributed by the oxygen atoms is -8.

This means that the oxidation number of Mn in 2MnO4- must be +6 in order to balance the overall charge.