1.  (78.92 u)(0.5054) + (80.92 u)(0.4946) = 79.91 u
2. (24 u) (0.7870) + (25 u)(0.1013) + (26 u) (0.1117) = 24.32 u
3. bromine: 79.90 u
   magnesium: 23.32 u
   almost the same

(source)