c
$P = nkT$
$n =\frac{ P }{ kT }=\frac{h dg }{ kT }$
Number of molecules in the electron tube is $= n \times V =\frac{ hdgV }{ kT }$
$\Rightarrow n =\frac{ hdg }{ kT }=\frac{1.2 \times 10^{-10} \times 13600 \times 9.8 \times 100 \times 10^{-6}}{1.38 \times 10^{-23} \times 300}=3.86 \times 10^{11}$