Chapter 4 - Atomic Structure - 4.3 Distinguishing Among Atoms - Sample Problem 4.5 - Page 119: 24

63.5 amu

Multiply the percent of the relative abundance of each isotope by its atomic mass, and add them get the atomic mass. Atomic mass of isotope 1 = 62.93 amu (69.2%) Atomic mass of isotope 2 = 64.93 amu (30.8%) 62.93 (0.692) + 64.93 (0.308) = 63.5 amu