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
A ticket for a round-trip in which the departing flight and the returning flight are separated by several months will cost Professor Popkin more than a ticket for a round-trip in which they are separated by less than one week.