C6H6 +Br2---->C6H5Br +HBr
n(C6H6)=m/M=15.6/78= 0.2 моль
n(C6H6)=n(C6H5Br)
m(C6H5Br)= n XM= 0.2 x 157=31.4г
30/31.4= 0.955 или 95.5 %