Solution 8.7.4.1.
Using the given information, apply the gamma distribution. Since 10 arrive on average in one hour then you would expect 1 to arrive in 6 minutes and therefore 3 to arrive on average in 18 minutes.
With \(\mu = 18\) minutes,
\begin{equation*}
P(X \lt 20) = F(20).
\end{equation*}