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.
Magnesium 24, 25, and 26 are stable with abundances of 89%,10%, and 11% respectively. Much more information is available at Nuclear structure and decay data. Views.