To determine the oxidation number of Mn in MnO4-, we can follow these steps:

1. Assign the oxidation number of oxygen (-2) to each oxygen atom in the compound. Since there are four oxygen atoms, the total oxidation number contributed by oxygen is -2 x 4 = -8.

2. Let x be the oxidation number of manganese (Mn) in MnO4-.

3. The overall charge of the MnO4- ion is -1.

4. We can set up the equation to balance the total oxidation numbers in the compound:

x + (-8) = -1

5. Solving for x, we get:

x = -1 + 8 = +7

Therefore, the oxidation number of manganese (Mn) in MnO4- is +7.