Atomic mass of Br = 79.904
Now lets say y% is abundance of 79Br.
Then abundance of 81Br = (100 - y)
mass due to 79Br = 78.9183 * y/100 = 0.789183y
mass due to 81Br = 80.9163 x (100 - y)/100 = 0.809163(100 - y)
Therfore
0.789183y+ 0.809163(100 - y) = 79.904
0.789183y + 80.9163 - 0.809163y = 79.904
- 0.01998y= 79.904 - 80.9163
= - 1.0123
y = 1.0123/0.01998 = 50.67%
79Br = 50.67%
now
81Br = 100 - 50.67 = 49.33%
hope this helps
