N non-interacting molecules are distributed among three non-degenerate energy levels $\varepsilon_{0}=0, \varepsilon_{1}=1.38\times10^{21}\,\mathrm{J}$ and $\varepsilon_{2} \; = \; 2.76\times10^{-21}\,\mathrm{J}$, at 100 K. If the average total energy of the system at this temperature is $1.38\times10^{-18}\,\mathrm{J}$, the number of molecules in the system is: