#### Answer

(Isotopic Mass of Br-79)(Abundance of Br-79) + (Isotopic Mass of Br-81) (Abundance of Br-81) = Atomic Mass of Bromine
Suppose x = Abundance of Br-79
This implies that Abundance of Br-81 = 1-x
Therefore
(78.9183) (x) + (80.9163)(1-x) = 79.904
78.9183x + 80.9163 – 80.9163x = 79.904
-1.998x = -1.0123
x = 0.50667
1-x = 1 - 0.5067
1-x = 0.4933
Hence %age of Br-79 = 50.667%
And %age of Br-81 = 49.33%

#### Work Step by Step

As given in answer