The Element Bromine Has Two Main Isotopes That Result in Almost

Short Answer

The element bromine has two main isotopes that result in almost all bromine on Earth. The two are listed below with their atomic masses. The average atomic mass of bromine is listed as 79.904 u. Using this information what is the relative abundances of the two chlorine isotopes?
Bromine-79: 78.918 u
Bromine-81: 80.916 u ,Hint: Set up a weighted average calculation where one percentage is x and the other percentage is (100 - x).