Rules to remember:

* Oxygen usually has an oxidation number of -2 (except in peroxides).

* The sum of the oxidation numbers in a neutral molecule is zero.

Steps:

1. Assign oxygen its usual oxidation number: Each oxygen atom in NO2 has an oxidation number of -2.

2. Let 'x' be the oxidation number of nitrogen:

3. Set up an equation: x + 2(-2) = 0

4. Solve for 'x': x - 4 = 0

x = +4

Therefore, the oxidation number of nitrogen in NO2 is +4.