As it is clear from the values only that the average atomic mass of copper is 63.55 which is nearer to the atomic mass of isotope 63 cu, hence the percentage of 63 CU isotope will be more in copper.
The other way we can do the same is by calculating the percentage of each isotope.
Let the abundance of isotope 63 = x
The abundance of isotope 65 = 100-x
[tex]63.55 = \frac{63x + 65 (100-x)}{100}[/tex]
[tex]6355 = 63x + 65 (100-x)[/tex]
[tex]6355 = 63x +6500 - 65x[/tex]
2x = 145
x = 72.5
Hence percentage of 63 is 72.5%
