0.5 g of hydrocarbon gave 0.9 g water on combustion. The percentage of carbon in hydrocarbon is

$$\begin{gathered} \% \mathrm{of} \mathrm{H} = \frac{2}{18} \times \frac{\mathrm{wt}. \mathrm{of} {\mathrm{H}}_2\mathrm{O}}{\mathrm{wt}. \mathrm{of} \mathrm{organic} \mathrm{compound}} \times 100 \\ = \frac{2}{18} \times \frac{0.9}{0.5} \times 100 = 20\% \end{gathered}$$

Since percentage of hydrogen is 20. Therefore, remaining is carbon i.e. 80 %.