How is the mass of the atom calculated? by adding the number of protons and electrons
by adding the number of protons and neutrons
by subtracting the number of electrons from the number of protons
by adding the number of neutrons and electrons

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.

mass of a proton: 1.67262 × 10−27 kg

mass of a neutron: 1.67493×10−27 kg

mass of an electron: 9.10938 × 10-31 kg