Average mass of element X can be calculated by taking sum of masses of atoms in the element multiplied by their percentage.
Here, masses of atoms and their percentage is given as follows:
24.0 amu mass has 78.7 % abundance, 25.0 amu has 10.1% abundance and 26.0 amu has 11.2% abundance thus, atomic mass is:
[tex]Atomic mass=24\times \frac{78.7}{100}+25\times \frac{10.1}{100}+26\times \frac{11.2}{100}=24.325 amu[/tex]
Therefore, atomic mass of element X will be 24.325 amu.
