Answer: The average atomic mass of element X is 63.545 amu.
Explanation:
The average atomic mass is can be calculated by the summation of the product of the atomic mass of each isotope multiplied by its abundance (abundance % / 100.0)
Element X has two isotopes:
1) At.mass = 62.9296 amu and its abundance = 0.6915
2) At.mass = 64.9278 amu and its abundance = 0.3085
The average atomic mass of element X = (62.9296 amu)*(0.6915) + (64.9278 amu)*(0.3085) = 63.454 amu