Problem B. 4690. (February 2015)
B. 4690. 9 mathematicians of a university went to a conference together. Since the presentations were boring, they fell asleep several times, each of them at most 4 times. For any pair of mathematicians, there was a time when they were both sleeping. Show that there was a time instant when at least three of them were sleeping.
(5 pont)
Deadline expired on 10 March 2015.
Statistics:
82 students sent a solution.  
5 points:  56 students. 
4 points:  8 students. 
3 points:  3 students. 
2 points:  4 students. 
1 point:  6 students. 
0 point:  5 students. 
