$${C_x}{H_y} + \left( {x + {y \over 4}} \right){O_2} \to x\,C{O_2} + {y \over 2}{H_2}O$$

Given organic compound C_xH_y= 0.3 gm

Produced carbon dioxide (CO₂) = 0.2 gm

Produced water (H₂O) = 0.1 gm

Moles of CO₂ = $${{0.2} \over {44}}$$

$$\therefore$$ Moles of C atom = $${{0.2} \over {44}}$$

$$\therefore$$ Mass of C atom = $${{0.2} \over {44}}$$ $$\times$$ 12 = 0.0545

Moles of H₂O = $${{0.1} \over {18}}$$

$$\therefore$$ Moles of H atoms = $${{0.1} \over {18}}$$ $$\times$$ 2

$$\therefore$$ Mass of H atoms = $${{0.1} \times2\over {18}}$$ $$\times$$ 1 = 0.0111

$$\therefore$$ % of C atom = $${{0.0545} \over {0.3}}$$ $$\times$$ 100 = 18%