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.
All isotopes of an element will always have the same number of protons. The UPPER number represents the MASS NUMBER, which is number of protons PLUS number of neutrons. This is unique for every isotope. Magnesium 24 has 12 neutrons, Magnesium-26 has 14 neutrons and Magnesium-30 has 18 neutrons.