Reading the Rate Off the Quiet Windows

Support calls reach a help desk according to a Poisson process with a constant rate of $\lambda$ calls per hour. Long-run records show that in 30% of all half-hour windows, no call arrives at all.

What is the probability that exactly one call arrives in a full one-hour window?

Recall the Poisson PMF, where $\mu$ is the expected count for the window in question:

$P(N = k) = \frac{e^{-\mu}\,\mu^{k}}{k!}$

You may use $\ln(0.3) \approx -1.20397$.

Round to four decimal places.