The natural abundance of #"Br-81"# is #49.31%#.

This means that #49.31%# of all of the world's bromine #"Br"# exists in the form of #"Br-81"#.

As there is only one other isotope of bromine, #100% - 49.31% = 50.69%# of it exists in the form of #"Br-79"#.

Therefore, the natural abundance of #"Br-79"# is #50.69%#

The atomic mass of bromine is #79.904# #"amu"#.

#Rightarrow 79.904# #"amu"# #= 50.69% times x + 49.31% times 80.9163# #"amu"#

We need to solve for #x#, which in this case is the mass of #"Br-79"#:

#Rightarrow 79.904# #"amu"# #= 0.5069 x + 0.4931 times 80.9163# #"amu"#

#Rightarrow 79.904# #"amu"# #= 0.5069 x + 39.89982753# #"amu"#

#Rightarrow 40.00417247# #"amu"# #= 0.5069 x#

#Rightarrow 78.919259164# #"amu"# #= x#

#therefore x = 78.919# #"amu"#

Therefore, the mass of #"Br-79"# is #79.919# #"amu"#.