$C_3H_8\rightarrow C_3H_6+H_2$ $x x x$
$C_3H_8\rightarrow C_2H_4+CH_4$
$y y y$
$\Rightarrow \begin{cases}x+y=\frac{8,8.90}{44.100} \\ \frac{2x+16y+\frac{44.8,8.(100-90)}{44.100}}{x+y+\frac{8,8.(100-90)}{44.100}}=2.7,3 \end{cases}$
$\Rightarrow \begin{cases}x+y=0,18 \\\frac{2x+16y+0,88}{x+y+0,02}=14,6 \end{cases}\Rightarrow \begin{cases}x+y=0,18 \\ 12,6x-1,4y=0,588 \end{cases}$
$\Rightarrow \begin{cases}x=0,06 mol \\ y=0,12 mol \end{cases}\Rightarrow Z:\begin{cases}H_2:0,06 mol\\CH_4:0,12 mol\\C_3H_8:0,02 mol \end{cases}\Rightarrow \begin{cases}\%H_2=30\% \\\%CH_4=60\%\\\%C_3H_8=10\% \end{cases}$