please help explain how this is done

Calculate the number of neutrons that these isotopes possess. Their mass number A is indicated. Example: Mass number A= protons + neutrons Atomic number Z= protons Therefore, Number of neutrons = mass number A-atomic number Z So, for C12 the mass number is 12 (i.e. protons + neutrons =12 ). From the periodic table we find that the atomic number Z for C is 6 (i.e. the number of protons is 6 ). Therefore, 126=6 Thus, C12 has 6 protons. 1) C14 2) Mg25 3) Au197 4) Cu61 5) 0-17