# The atomic weight of Cu is 63.546.There are only two naturally occuring isotopes of copper,$^{63}Cu$ and $^{65}Cu$.The relative abundance of the $^{63}Cu$ isotope must be

$\begin{array}{1 1}(a)\;11.3\% \\ (b)\;31.5\% \\(c)\;52.4\% \\(d)\;72.7\% \end{array}$

Suppose the relative abundance of $Cu^{63}$ isotope=$x$
$63.546=\large\frac{63\times x+65(100-x)}{100}$
$x=72.7$
Hence (d) 72.7% is the correct answer.