20 ml of a mixture of ${\mathrm{CO}}_2$ and ${\mathrm{C}}_2{\mathrm{H}}_4$ was mixed with excess of ${\mathrm{O}}_2$ gas and was exploded. On bringing the solution back to the original room temperature a contraction of 12 ml was observed. What is the volume percentage of ${\mathrm{CO}}_2$ in the original mixture?