Explanation

(b) The pressure exerted by the gas,
$p = \frac{1}{3}\rho c^{2}$
$\ \ \ \ = \frac{1}{3}\frac{m}{V}{\overline{c}}^{2}$
$\ \ \ \ = \frac{2}{3}\left( \frac{1}{2}m{\overline{c}}^{2} \right)$
$(\because\ \frac{1}{2}m{\overline{c}}^{2} = \frac{E}{V} = energy\ per\ unit\ volume,\ V = 1)\ $
$p = \frac{2}{3}E$