If the natural abundance of Br-79 is 50.69 %, what is the natural abundance of Br-81?

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%