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 :
