When $\alpha$ is a positive integer $n$, the gamma distribution is also known as the Erlang distribution.
Setting $\mathrm{a}=n$ in the gamma distribution on page $195,$ the Erlang distribution is $f(x)=\left\{\begin{array}{ll}\frac{x^{n-1} e^{-n / 3}}{\beta^{n}(n-1) !^{\prime}}, & x>0 \\ 0, & \text { elsewhere }\end{array}\right.$
It can be shown that if the times between successive events are independent, each having an exponential distribution with parameter $3,$ then the total elapsed waiting time $X$ until all $n$ events occur has the Erlang distribution. Referring to Review Exercise $6.60,$ what is the probability that the next 3 calls will be received within the next 30 minutes?