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