I got another good puzzle to think about:
80 students are enrolled in 2 courses that scheduale on thursday every week in the same hours.
there are 14 weeks (14 courses)
in each week, some of the students come to course A and all the rest go to course B.
A secretery check provide something unusual:
for each 2 weeks (not necessarily consecutive weeks, but even those), exactly 40 of the students are changed their mind.
(for example: First week: 60 to A, and 20 to B. Third week: 20 to A, and 60 to B.)
The teacher of course A, ask for a list of the students that have been participate in his class for exactly 7 weeks (and so exactly 7 weeks for course B), and he got a list of 78 students (of 80 students)
i cannot show an example of the problem that meet with it.