There are two naturally occurring isotopes of bromine. 79br has a mass of 78.9183 amu. 81br has a mass of 80.9163 amu. determine the abundance of each isotope.

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

C. 1s22s22p63s23p43d104s24p5