\(n_{Br_2}=\dfrac{8}{160}=0.05\left(mol\right)\)
\(C_2H_4+Br_2\rightarrow C_2H_4Br_2\)
\(0.05......0.05\)
\(V_{C_2H_4}=0.05\cdot22.4=1.12\left(l\right)\)
\(V_{CH_4}=20-1.12=18.88\left(l\right)\left(mol\right)\)
\(\%V_{C_2H_4}=\dfrac{1.12}{20}\cdot100\%=5.6\%\)
\(\%V_{CH_4}=100-5.6=94.4\%\)