Question

The element nitrogen has the atomic number 7 and an atomic mass of 14. How many neutrons does an atom of nitrogen contain?

Answer 1

(Atomic Mass) - (Atomic Number) = (# of neutrons)

This is of course a simplification... it is possible for certain (exotic) atoms to break this rule. Heavy water is the first that comes to mind. However, if you're only looking at introductory material to learning atomic structure, this equation is suffice for most purposes.