20 mL of methane is completely burnt using 50 mL of oxygen. The volume of the gas left after cooling to room temperature is

$C{H}_4\left(g\right)+2O_2\left(g\right)\to C{O}_2\left(g\right)+2H_{2}O\left(l\right)$

given  20ml 50 ml
Vol.  20ml   40 ml  20ml

involved

_____________________
10ml 20ml
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Total Vol. 10 ml (unreacted $\left.{\mathrm{O}}_2\right)+20\mathrm{ml}$ (vol.of $\left.{\mathrm{CO}}_2\right)$

= 30 ml