It contains 24 neutrons.
Explanation:
Mass number = Protons + Neutrons
If you were to have 24 neutrons, the mass number would be 36, invalidating the known information. You have 12 protons and 12 neutrons to form a mass number of 24. Remember that your protons are the same thing as the atomic number, so you can make an equation using your known information to derive true statements.