$X:\begin{cases}H_2:x mol \\C_2H_4:y mol \end{cases}$
$H_2+C_2H_4\rightarrow C_2H_6$
$a a a$
$\Rightarrow \begin{cases}\frac{2x+28y}{x+y}=4.3,75 \\ \frac{2x+28y}{x+y-a}=4.5 \end{cases}\Rightarrow \begin{cases}x=y \\ y=2a \end{cases}\Rightarrow H=\frac{a.100}{x}=50\%$