Calculate the average atomic mass of Mg if the naturally occurring isotopes are 24Mg, with an exact atomic mass of 23.98504 amu and a percent abundance of 78.70%, 25Mg, with an exact atomic mass of 24.98584 amu and a percent abundance of 10.13%, and 26Mg, with an exact atomic mass of 25.98259 amu and a percent abundance of 11.17%