So let's say the first bromine isotope is Br-1 and the second one is Br-2.
For Br-1 and Br-2 we will have to divide the percentage by 100.
So Br-1 will go from 50.69% to 0.5069.
And Br-2 will go from 49.31 to 0.4931.
For Br-1 you need to multiply abundance of 78.2 and 0.5069 which equals 39.63958.
And then multiply 80.92 and 0.4931 for Br-2, which then equals 39.901652.
Now you add the totals you got which are 39.63958 + 39.901652 and that equals 79.541232, which is the total atomic mass
