Combustion mcq

Using pV = nRT, moles of gas is proportional to volume, so you can figure out 1 mole of hydrocarbon needs 6 moles of oxygen so 1 x (2c + 0.5h) = 6 x (2), and you can just try the formulae given and see which one works.

That weird looking formula is because every mole of C needs two moles of O to combust and every mole of H needs half a mole of O to combust (to make CO2 and H2O). Of course every mole of O2 has two moles of O.

I don't get it :/
Where did you get (2c + 0.5h) from?

Ignore my previous message, it's not very clear.
If you were to make a general formula for complete combustion of a hydrocarbon:

You know what n should be from the ratio of the volumes of the two gases (from pV = nRT, n = 6). By considering the amount of oxygen on each side of the equation you can find n in terms of a and b, so you can just try the values given until you get 6. (in the multiple choice a is always 4 so you just have to think about b)