The atomic mass of an element is the weighted average mass of the differente isotopes of the element.
Given that there are only two isotopes, you can do the following formula:
atomic mass = % abundance of isotope 1 * atomic mass of isotope 1 + % of abundance of isotopoe 2 * atomic mass of isotope 2
call x the % of abundance of isotope 1, then 100 - x is the abundance of the other isotope
63.55 = (x/100)*63 + [(100 -x) / 100 ]* 65 = 0.63x + 65 - 0.65x
63.55 - 65 = 0.63x - 0.65x
-1.45 = - 0.02x
x = 1.45/0.02 = 72.5%
Answer: 72.5% is the approximate natural abundance of 63 Cu
