Issued as a cumulative or non-cumulative L/C, the former allows for unused credit amounts to rollover into subsequent periods while the latter maintains a fixed amount of credit available each period.
average atomic mass of magnesium is 24.3 g/mole so 24.3 X 1.56 X1021 / 6.02 X1023 = .063 grams.
The isotope chart will show that there are three stable isotopes of magnesium: Mg-24, Mg-25, and Mg-26. The number is the mass number. It is the total number of protons and neutrons in the atom. Subtract the atomic number from the mass number and you get the number of neutrons.