Br-79 (am=78.92amu)br-81(am=80.92). The avge atomic m of br=79.904amu. Find abundances (%) of isotopes.?

Bromine consists of two isotopes br-79 (atomic m=78.92amu) and br-81(atomic m=80.92amu). The average atomic m of Bromine is 79.904amu. Determine the abundances (percentages) of these isotopes.