$13-3.$ The Lower Colorado River Authority $($LCRA$)$ has been studying congestion at the boat$-$launching

ramp near Mansfield Dam. On weekends, the arrival rate averages five boaters per hour, Poisson

distributed. The average time to launch or retrieve a boat is $10$ minutes, with negative exponential

distribution. Assume that only one boat can be launched or retrieved at a time.

The LCRA plans to add another ramp when the average turnaround time $($i$.$e$.,$ time in the system$)$

exceeds $90$ minutes. At what average arrival rate per hour should the LCRA begin to consider adding

another ramp?

If there were room to park only two boats at the top of the ramp in preparation for launching, how

often would an arrival find insufficient parking space?