$a/PTHH :$
$CH_4+2O_2\overset{t^o}\to CO_2+2H_2O$
$C_2H_4+3O_2\overset{t^o}\to 2CO_2+2H_2O$
b/Gọi $V_{CH_4}=a;V_{C_2H_4}=b$
Ta có :
$V_{hh}=a+b=20,16$
$V_{CO_2}=a+2b=35,84$
Ta có hpt :
$\left\{\begin{matrix}
a+b=20,16 & \\
a+2b=35,84 &
\end{matrix}\right.$
$⇔\left\{\begin{matrix}
a=4,48 & \\
b=15,68 &
\end{matrix}\right.$
$⇒V_{CH_4}=4,48l;V_{C_2H_4}=15,68l$
$c/V_{O_2}=2a+3b=56l$