Answer
The relative sizes represent the relative abundances of each isotope of $Br_2$ (notice how the relative sizes sum to 1). Therefore, we may use the relative sizes as abundance values to calculate the average atomic mass.
Work Step by Step
Notice that the sum of the relative sizes equals 1.
0.2534+0.5000=0.2466=1.000
Therefore, this gives a clue in that the relative sizes are in fact the relative abundances of each isotope. Therefore, we may use this data to calculate the average atomic mass as we would with relative abundances.
