Greydog Bus Company operates buses between Boston and Washington, D.C. A bus trip between these two cities takes 6 hours. Federal law requires that a driver rest for four or more hours between trips....

Greydog Bus Company operates buses between Boston and Washington, D.C. A bus trip between these two cities takes 6 hours. Federal law requires that a driver rest for four or more hours between trips. A driver’s workday consists of two trips: one from Boston to Washington and one from Washington to Boston. Table gives the departure times for the buses.

Greydog’s goal is to minimize the total downtime for all drivers. How should Greydog assign crews to trips? Note: It is permissible for a driver’s “day” to overlap midnight. For example, a Washington-based driver can be assigned to the Washington–Boston 3 P.M. trip and the Boston–Washington 6 A.M. trip.