Vehicles begin to arrive at a park entrance at 7:45 A. M. at a constant rate of six per minute and at a constant rate of four vehicles per minute from 8:00 A. M. on. The park opens at 8:00 A. M. and the manager wants to set the departure rate so that the average delay per vehicle is no greater than 9 minutes (measured from the time of the first arrival until the total queue clears). Assuming D/D/1 queuing, what is the minimum departure rate needed to achieve this?