Assuming that $W s=2$ seconds and $\rho=0.8$ find the numerical values of $E\left[N_b\right], E\left[N_b^2\right]$, and $\sigma_{N_b}^2$ for
(a) an $\mathrm{M} / \mathrm{M} / 1$ queueing system,
(b) an $\mathrm{M} / \mathrm{E}_4 / 1$ queueing system, and (c) an M/D/1 queueing system.