If naturally occurring Iron has a percent abundances of 80% Fe-56 atoms and 20% Fe-55 atoms. What is it's atomic mass?

Because electrons have negligible mass, to account for the mass of the isotope, there must be 16 neutrons, 16 neutrally charged, massive, fundamental particles present in the sulfur nucleus. and thus finally, if we have the 32s2− ion, there are 18 electrons, 16 protons, and 16 neutrons.

The answer is letter c