Suppose the arrival of telephone calls at a cell tower can be modelled as a Poisson PMF. That is, if X is the number of calls that arrive in $t$ minutes, then
$\Pr (X = k) = \frac{(\lambda t)^k}{k!}e^{-\lambda t}$, $k = 0, 1, 2, \dots$
where $\lambda$ is the average arrival rate in calls/minute. Suppose that the average rate of calls is $\lambda = 1$ calls/minute. What is the probability that there are fewer than 5 calls in the first 2 minutes?