At $300\;K$ and $1\;atm, 15\; mL$ of a gaseous hydrocarbon requires $375\; mL$ air containing $20\%$ $O_2$ by volume
for complete combustion. After combustion the gases occupy $330\; mL$. Assuming that the water formed is in liquid form and the volumes were measured at the same temperature and pressure, the formula of the hydrocarbon is
