add the number of protons to the number of electrons add the number of protons to the number of neutrons subtract the number of protons from the number of neutrons subtract the number of electrons from the sum of protons and neutrons
I gave my students this multiple choice question: How is the mass of an atom calculated?

Only one answer is correct among these 4 choices:
- add the number of protons to the number of electrons
- add the number of protons to the number of neutrons
- subtract the number of protons from the number of neutrons
- subtract the number of electrons from the sum of protons and neutrons

Could you tell me which one is correct?

add the number of protons to the number of neutrons