First, state the balanced equation that represents the combustion:
CH4 + 2O2 → CO2 + 2H2O
Second, state the molar ratios: 1 mol CH4 : 1 mol CO2.
Third, given that at constant temperature and pressure, the volume of an ideal gas is proportinal to the number of moles, you can state the volumetric proportion:
People had asked this many times and that is why they came up with methods and standards that will answer these type of questions. You can look it up in the NIST or the National Institute for Standards and Technology.
