isotopes are atoms of the same element with different number of neutrons, hence different mass numbers.
relative atomic mass is the weighted average atomic masses of the isotopes.
atomic mass of element z =78.918 amu x (100-49.31)%+ 80.916 amu x 49.31%
atomic mass = (78.918 x 50.69%) + (80.916 x 49.31%)
= 40.00 + 39.90
atomic mass of element z = 79.90
