Copper has two naturally occurring isotopes, 63cu(isotopic mass 62.9296 amu) and 65cu(isotopic mass 64.9278 amu). of copper has an atomic mass of 63.546 amu, what is the percent abundance of each isotope

Let x = % abundance 63Cu 
let y = % abundance 65Cu 
x +y = 100 
y = 100-x 

63.546 = [ 62.9396 x + 64.9278 ( 100-x) ] / 100 

6354.6 = 62.9396 x + 6492.78 - 64.9278 x 

138.18 = 1.9882 x 

x = 69.50 % 
y = 100 - 69.50 = 30.50 %