The atomic mass of copper is 63.5u. It exists in the form of two isotopes Cu 63 and Cu 69. Calculate the percentage of each isotope 

copper has two naturally occurring isotopes, copper-63 and copper-65. This means that their respective decimal abundance must add up to give 1.

If you take x to be the decimal abundance of copper-63, you can say that the decimal abundance of copper-65 will be equal to 1−x.

Therefore, 

Copper-63x⋅62.9296u+ copper-65(1−x)⋅64.9278u=63.546u

Solve this equation for x to get

62.9296⋅x−64.9278⋅x=63.546−64.9278

1.9982⋅x=1.3818⇒x=1.38181.9982=0.69152

This means that the percent abundances of the two isotopes will be

● 69.152%→63Cu

● 30.848%→65Cu