When $20\mathrm{ml}$ of methane and $20\mathrm{ml}$ of oxygen are exploded together and the reaction mixture is cooled to laboratory temperature. The resulting volume of the mixture is

Balanced reaction of combustion of methane 

            ${\mathrm{CH}}_4+2{\mathrm{O}}_2\to {\mathrm{CO}}_2+2{\mathrm{H}}_2\mathrm{O}$

Balanced reaction of combustion of methane 

           ${\mathrm{CH}}_4+2{\mathrm{O}}_2\to {\mathrm{CO}}_2+2{\mathrm{H}}_2\mathrm{O}$

Here limiting reagent is Oxygen as its ratio is less.

Hence $20\mathrm{ml}$ of Oxygen will require $10\mathrm{ml}$ of methane to react.

Hence $10\mathrm{ml}$ of Carbon dioxide is formed and $20\mathrm{ml}$ of water is formed and $10 \mathrm{ml}$of methane is remaining. 

So total volume will be $10+10+20=40\mathrm{ml}$