A certain element (X) has isotopes with mass numbers of 10
and 11. The X-10 isotope has an atomic mass of 10.013, and it
represents 19.9% of all atoms of this element. The X-11
isotope has an atomic mass of 11.009, and it represents 80.1%
of all atoms of this element. Therefore, the average atomic
mass, or atomic weight, of this element is
amu.