Let x g of each gas is mixed. Mole of ethane$=\frac{x}{30}$ $\, \, \, \, \, \, \, \, Mole of hydrogen =\frac{x}{2}$ Mole fraction of hydrogen$=\frac{\frac{x}{2}}{\frac{x}{2}+\frac{x}{30}}=\frac{15}{16}$ $\Rightarrow \, \, \, \frac{Partial\, pressure\, of\, H_2}{Total\, pressure}=$Mole fraction of hydrogen