Rate of diffusion depends upon the mass of the gas as $r \propto \large\frac{1}{\sqrt m}$

Also, $x = $ Rate of Diffusion of $H_2 \times t$ and $1-x =$ Rate of Diffusion of $N_2 \times t$

$\Rightarrow \large\frac{x}{r_{H_2}}$ $= \large\frac{1-x}{r_{N_2}}$

$\Rightarrow \large\frac{x}{1-x}$ $= \large\frac{r_{H_2}}{r_{N_2}}$

$\Rightarrow \large\frac{x}{1-x}$ $= \large\frac{\sqrt 28}{\sqrt 2}$

$\Rightarrow x = 0.79$