Question 24
A teacher has a class of twelve students. She thinks it would be a nice idea if they change desks every day, so she has painted arrows on the floor from desk to desk. Each desk has one arrow going to it and another going from it. Each morning, the students pick up their books and move to the desk indicated by the arrow. By choosing her arrows carefully, the teacher has arranged it so that the longest possible time will pass before all the students are back in their original desks at the same time. How many days is that?