\textbf{166.} In a container with a volume of $33.6$ liters, a mixture of oxygen and helium gases exists. The gas pressure is $2\times10^5$ pascals and the temperature is $7$ degrees Celsius. If the mass of the gas is $54$ grams, what percentage of the molecules are oxygen?
$$\left(M_{He}=4\,\frac{g}{mol},\; M_{O_2}=32\,\frac{g}{mol},\; R=8\,\frac{J}{mol\cdot K}\right)$$
\begin{itemize}
\item[(1)] $50$
\item[(2)] $60$
\item[(3)] $40$
\item[(4)] $30$
\end{itemize}