How do the number of protons number of neutrons and a mass number relate to each other

The mass number is the sum of protons and neutrons in an atom.

The nucleus makes most of the an atom and in the nucleus are the protons and neutrons. Although atoms are also made up of electrons, they are so small that the mass of an electron is negligible. So the mass of an atom is determined by the protons and neutrons instead.

Example:

An oxygen atom has an atomic number of 8 and has a mass number of 16.

The atomic number indicates also th number of protons. Since the atomic number of oxygen is 8, then it has 8 protons.

So if the mas number of oxygen is 16 and the number of protons is 8, this wold mean that an Oxygen atom has 8 neutrons.

Mass Number = Protons + Neutrons

16 = 8 + n

16-8 = n

8 = n