percents abundance and isotopes

Question

Copper has two naturally occurring isotopes and an atomic mass of 63.546 amu. Cu-63 has a mass of 62.940 amu and an abundance of 69.17%. What is the identity and percent abundance of copper's other isotope? 

Answer 1

Since one isotope has an abundance of 69.17%, the other must have an abundance of 100% - 69.17% = 30.83% = 0.3083 as a decimal.

(62.940)(0.6917) + (0.3083)(x) = 63.546

43.5356 + 0.3083x = 63.546

0.3083x = 20.01

x = atomic mass 20.01 present at 30.83%