Chemistry
posted by Anonymous .
Theatomic mass of copper is 63.55.There are only two isotopes of copper , Cu63 with a mass of 62.93 amu and Cu65 w/ a mass of 64.93 amu. What is the percent of each of these two isotopes?

Total is 100%, so assuming x is the fraction of Cu63, (1x) is the fraction of Cu65.
Atomic weight is the (weighted) average of all isotopes.
Therefore you have:
x*62.93 + (1x)*64.93 = 63.55
x = 0.69.
So you have 69% Cu63 and 31% Cu65.