A periodic table might list the average atomic mass of magnesium as being 24.312 u. That average value is the result of weighting the atomic masses of the magnesium isotopes according to their natural abundances on Earth. The three isotopes and their masses are 24Mg (23.985 04 u), 25Mg (24.985 84 u), and 26Mg (25.982 59 u). The natural abundance of 24Mg is 78.99 % by mass (that is, 78.99 % of the mass of a naturally occurring sample of magnesium is due to the presence of 24Mg). What is the abundance of?

(a) 25Mg and

(b) 26Mg