Calculate the atomic mass of bromine. The two isotopes of bromine have atomic masses and relative abundance of 78.2 (50.69%) AMU and 80.92 AMU (49.31%)

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