Calculate the atomic mass for zinc given the following data for its natural isotopes:
64Zn 63.929 amu 48.89 %
66Zn 65.926 amu 27.81 %
67Zn 66.927 amu 4.11 %
68Zn 67.925 amu 18.57 %
70Zn 69.925 amu 0.62 %