Complete combustion of 750 g of an organic compound provides 420 g of CO2 d 210 g of H2O.

$44 \mathrm{gm} \mathrm{CO}_{2}$ have $12 \mathrm{gm}$ carbon

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

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

$\Rightarrow 114.545$ gram carbon

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

$\simeq 15.3 \%$

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

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

$=23.33 \mathrm{gm} \mathrm{H}_{2}$

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

$\approx 3 \%$