If the abundance of the first isotope is x, the abundance of the second isotope is 1-x.
Substituting into the atomic mass formula,
[tex]79.908=79x+81(1-x)\\\\79.908=81-2x\\\\-1.092=-2x\\\\x=0.546[/tex]
This means the abundance of Bromine-79 is 54.6%, and the abundance of Bromine-81 is 45.4%.
