Answer
True
Why
Let x be the atomic mass of the 2nd isotope
The isotopes occur in the proportion:
Cu-63 with atomic mass 62.9296 amu: 69.15%
Cu-x: 100–69.15= 30.85%
63.546=((62.9296*69.15)+(x*30.85))÷100
63.546=43.5158184+0.3085x
0.3085x= 63.546-43.5158184
x= 20.0301816÷0.3085
x=64.9276 amu
