In BrO3-, the oxidation number of Br can be determined as follows:

Let the oxidation number of Br be x.

The overall charge of BrO3- is -1.

The oxidation number of oxygen is -2.

There are three oxygen atoms in BrO3-.

So, the total oxidation number contributed by oxygen is 3 × (-2) = -6.

Therefore, the oxidation number of Br (x) can be calculated as follows:

x + (-6) = -1

x = -1 + 6

x = +5

Hence, the oxidation number of Br in BrO3- is +5.