To find the atomic mass of copper we have to multiply the atomic mass of each isotope times its corresponding abundance and then add them.
This way:
[tex]\begin{gathered} m=62.93amu\cdot0.6915+64.93amu\cdot30.85 \\ m=43.52amu+20.03amu \\ m=63.55amu \end{gathered}[/tex]
It means that the correct answer is 63.55 amu.
