$1)A:\begin{cases}H_2:x mol \\C_nH_{2n}:y mol \end{cases}$$*)\frac{D_{A/He}}{D_{B/He}}=\frac{n_B}{n_A}=\frac{7,5}{14,5}<\frac{1}{2}\Rightarrow x<y$
$H_2+C_nH_{2n}\rightarrow C_nH_{2n+2}$
$x x x$
$\Rightarrow \begin{cases}\frac{2x+14ny}{x+y}=4.7,5 (1) \\\frac{2x+14ny}{y}=4.14,5 (2)\end{cases}$
$*)\begin{cases}(1)\\ (2) \end{cases}\Rightarrow \frac{y}{x+y}=\frac{7,5}{14,5}\Rightarrow \frac{y}{x}=\frac{7,5}{14,5-7,5}\Rightarrow y=\frac{15x}{14}$
$(2)\Rightarrow \frac{2x+15nx}{\frac{15x}{14}}=58\Rightarrow n=4\Rightarrow C_4H_8$