Given: The density of the gaseous mixture of Hydrogen and Carbon-di-Oxide is $\large\frac{30\;g}{22.4\;l}$ at NTP

Let $x$ be the percent composition of Hydrogen.

Molar mass of mixture of $22.4\;l$ is $30g$, therefore:

$30 = \large\frac{x\times 2 + (100-x) \times 44}{100}$

$\Rightarrow$ solving for $x, x = \large\frac{1400}{44} $$= 33.33\text{%} \rightarrow 100 - x = 66.66\text{%}$