The Volume of oxygen needed for the complete combustion of $25$ml of $C_x{H}_y$ is

${\mathrm{C}}_{\mathrm{x}}{\mathrm{H}}_{\mathrm{y}}$ +$4\text{x+y/4}$ O₂ $\to$x CO₂ +Y/2 H₂O

$1$ volume of ${\mathrm{C}}_{\mathrm{x}}{\mathrm{H}}_{\mathrm{y}}$ requires $4\text{x+y/4}$ volume of $O_2$ gas

Thus, $25$ml of ${\mathrm{C}}_{\mathrm{x}}{\mathrm{H}}_{\mathrm{y}}$ requires $25(\mathrm{x}+\mathrm{y}/4)$ volume of ${\mathrm{O}}_2$ gas.