The volume of oxygen necessary for the complete combustion of 20 litre of propane is :

In the following reaction,

${\mathrm{C}}_3{\mathrm{H}}_8+5{\mathrm{O}}_2\to 3{\mathrm{CO}}_2+4{\mathrm{H}}_2\mathrm{O}$

1 volume of ${\mathrm{C}}_3{\mathrm{H}}_8$ requires 5 volume of ${\mathrm{O}}_2$

20 L of  ${\mathrm{C}}_3{\mathrm{H}}_8$ requires $5\times 20=100 \mathrm{L}$ of ${\mathrm{O}}_2$.