$C_\overline{n}H_{2\overline{n}+2 } \overset{+O_2}{\rightarrow}\overline{n}CO_2+(\overline{n}+1)H_2O $ $x \overline{n}x $
$\Rightarrow \overline{n}x=\frac{57,2}{44}=1,3\Rightarrow \frac{19,2\overline{n} }{14\overline{n}+2 }=1,3\Rightarrow \overline{n}=2,6\Rightarrow \begin{cases}C_2H_6:a mol \\C_3H_8:b mol \end{cases} $
$\Rightarrow \begin{cases}30a+44b=19,2 \\\frac{2a+3b}{a+b}=2,6 \end{cases}\Rightarrow \begin{cases}a=0,2 mol \\b=0,3 mol \end{cases}\Rightarrow \begin{cases}\%m_{C_2H_6}=31,25\% \\\%m_{C_3H_8}=68,75\% \end{cases}$