Answer
$209^\circ \rm C$
Work Step by Step
We know that the thermal energy of an ideal gas is given by
$$E_{th}=nC_{\rm V}T$$
where $C_{\rm V}$ is given by $C_{\rm P}-C_{\rm V}=R$
Hence,
$$E_{th}=n(C_{\rm P}-R)T$$
So the temperature of the gas is then given by
$$T=\dfrac{E_{th}}{n(C_{\rm P}-R)}\tag 1$$
We also know that the number of atoms of molecules of an ideal gas is given by $N=nN_{\rm A}$; where $N_{\rm A}$ is Avogadro's number, and hence $n=N/N_{\rm A}$.
Plug that into (1);
$$T=\dfrac{N_{\rm A}E_{th}}{N(C_{\rm P}-R)} $$
Plugging the known;
$$T=\dfrac{(6.022\times 10^{23})(1)}{(1\times 10^{20})(20.8-8.31)} $$
$$T=\color{red}{\bf 482}\;\rm K=\color{red}{\bf 209}^\circ C$$
