\(C_2H_4 + Br_2 \to C_2H_4Br_2\\ n_{C_2H_4} = n_{C_2H_4Br_2} = \dfrac{18,8}{188} = 0,1(mol)\\ \Rightarrow n_{CH_4} = \dfrac{6,72}{22,4} - 0,1 = 0,2(mol)\\ \%V_{C_2H_4} = \dfrac{0,1.22,4}{6,72}.100\% = 33,33\%\\ \%V_{CH_4} = 100\% - 33,33\% = 66,67\%\)