The natural abundance of bromine in general in nature is 100%.
This means that:
the natural abundance of Br-79 + the natural abundance of Br-80 = 100
We know that the natural abundance of Br-79 is 50.69%. This means that the natural abundance of Br-80 is the rest of the 100%.
Natural abundance of Br-80 = 100% - 50.69% = 49.31%