the balanced equation for the formation of methane is as follows;
C + 2H₂ --> CH₄
stoichiometry of H₂ to CH₄ is 2:1
for gases, at constant temperature and pressure, the volume of gas is directly proportional to number of moles.
Therefore stoichiometry of the equation is applied to volume ratio.
Therefore volume ratio of H₂ to CH₄ is 2:1
If 1 L of methane is formed - when 2 mol of H₂ reacts
when 20 L of methane is formed - 2/1 x 20 = 40 L of H₂ is required