Question

how is the mass number found

Answer 1

The mass number is the number of protons + the number of neutrons. You can find this on the periodic table

Answer 2

The mass number (A), also called atomic mass number or nucleon number, is the total number of protons and neutrons (together known as nucleons) in an atomic nucleus. Because protons and neutrons both are baryons, the mass number A is identical with the baryon number B as of the nucleus as of the whole atom or ion.

Answer 3

@radcliff69 cite your sources specially when directly copying-and-pasting, you cannot pass off other's people writing as your own.

http://en.wikipedia.org/wiki/Mass_number