It takes a plane 1 hour to fly between Washington DC and Boston and there is a turnaround time for every plane of 30 minutes at both Washington DC and Boston. Planes must depart from Washington DC (for Boston) at 7:00, 8:15, 13:00, 14:00 and 16:30 and from Boston (for Washington DC) at 9:30, 10:00, 13:30, 15:00 and 16:00. Planes may be reused throughout the day. Find the minimum number of planes that must start at Washington DC and Boston so that a plane can depart at every flight time.
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