Suppose that a fictitious element, x, has two isotopes: 59x (59.015 amu) and 62x (62.011 amu). the lighter isotope has an abundance of 71.7%. calculate the atomic mass of the element x.

The atomic mass of the element would simply be equal to the sum of the weighted average of each isotope, that is:

atomic mass = 59.015 amu * 0.717 + 62.011 amu * (1 - 0.717)

atomic mass = 59.863 amu