This semester Professor Popkin will commute to Montreal by plane, leaving every Tuesday and returning to Toronto every Friday
She instructs her travel agent to schedule for her one round-trip that departs from Toronto on the first Tuesday and returns to Toronto on the last Friday of the semester and additional round-trips that depart from and return to Montreal for each weekly commute in between
Professor Popkin's round-trip tickets are least expensive if each trip is scheduled so that at least one Saturday night intervenes between the departing flight and the returning flight.