Chemistry and Chemical Reactivity (9th Edition)

$0.781\,\mu g$
Time elapsed=63.5 hours=$\frac{63.5\,hours}{12.7\,hours}$ half-lives= 5 half-lives. The amount of Cu-64 has decreased to $(\frac{1}{2}\times\frac{1}{2}\times\frac{1}{2}\times\frac{1}{2}\times\frac{1}{2})$ of the original amount. The amount of Cu-64 remaining$=25.0\,\mu g\times(\frac{1}{2})^{5}=25.0\,\mu g\times\frac{1}{32}=0.781\,\mu g$