Answer: By adding the number of protons and neutrons
Explanation:
Protons and neutrons have very similar masses and are used in calculating the mass of an atom. One would calculate the mass of an atom by doing the (mass of a proton) x (# of protons) + (mass of a neutron) x (# of neutrons). Electrons have such a small mass that their mass is considered negligible and therefore is not included in mass calculations for atoms.