So, $420 gm CO _{2} \Rightarrow \frac{12}{44} \times 420$

$\Rightarrow \frac{1260}{11} gm$ carbon

$\Rightarrow 114.545$ gram carbon

So, $\%$ of carbon $=\frac{114.545}{750} \times 100$

$=15.3 \%$

$18 gm H _{2} O \Rightarrow 2 gm H _{2}$

$210 gm \Rightarrow \frac{2}{18} \times 210$

$=23.33 gm H _{2}$

$So , \% H _{2} \Rightarrow \frac{23.33}{750} \times 100=3.11 \%$

$\approx 3 \%$