Dear Alfiya, to be precise the abundance of Br-81 is 49.31%. But if you are asking for for 49.46% then the result slightly varies. So here it goes-

If 81 has abundance of 49.46%, then Br-79 must be 100-49.46=50.54. Then, using the average molecular mass formula-

80.916(0.4946) + x(100-49.46) = 79.904. Solve for x.

All the best. 

Hope this helps.