$\begin{aligned} & E=\text { Average Kinetic energy per unit volume }= \\ & {\left[\left\{(1 / 2) M V^2 r m s\right\} / V\right]} \\ & =(1 / 2) \times(M / V) \times V_{r m s}^2 \\ & \therefore E=(1 / 2) \rho V_{r m s}{ }^2(1) \\ & \text { Pressure }=P=(1 / 3) \rho V_{r m s}{ }^2 \text { hence } \rho \cdot V_{r m s}{ }^2=3 \\ & \therefore \text { from }(1), E=(1 / 2) \times 3 P=(3 / 2) P \\ & \therefore P=(2 / 3) E\end{aligned}$