The atomic mass of an element is the average of the atomic masses of its isotopes. The relative abundance of isotopes must be taken into consideration, therefore: atomic mass of copper = atomic mass of isotope 1 * abundance 1 + atomic mass of isotope 2 * abundance 2
We know: atomic mass of copper = 63.546 amu The atomic mass of isotope 1 is: 62.939 amu The abundance of isotope 1 is: 69.17% = 0.6917 The atomic mass of isotope 1 is: x The abundance of isotope 2: 100% - 69.17% = 30.83% = 0.3083