Em tham khảo nha:

\(\begin{array}{l}
2{H_2} + {O_2} \xrightarrow{t^0} 2{H_2}O\\
C{H_4} + 2{O_2} \xrightarrow{t^0} C{O_2} + 2{H_2}O\\
{V_{{O_2}}} = 39,2 \times 20\%  = 7,84l\\
{n_{{O_2}}} = \dfrac{{7,84}}{{22,4}} = 0,35\,mol\\
hh:{H_2}(a\,mol),C{H_4}(b\,mol)\\
{n_{hh}} = \dfrac{{5,6}}{{22,4}} = 0,25\,mol\\
\left\{ \begin{array}{l}
a + b = 0,25\\
0,5a + 2b = 0,35
\end{array} \right.\\
 \Rightarrow a = 0,1;b = 0,15\\
\% {m_{{H_2}}} = \dfrac{{0,1 \times 2}}{{0,1 \times 2 + 0,15 \times 16}} \times 100\%  = 7,69\% \\
\% {m_{C{H_4}}} = 100 - 7,69 = 92,31\% 
\end{array}\)