The answer is 55%. If you were given the % of each isotope and asked to calculate the relative atomic mass, you would do the following
(79x55+81x45)/100 = 79.9
Now, in the question you are asked what is the % of Br-79, so replace with "x" (and replace the Br-81 % with "100-x" as both added together must equal 100%).
(79x + 81(100-x))/100 =79.9
79x + 81(100-x)=7990
79x +8100 -81x =7990
-2x=-110 x=55
Therefore Br-79 55%