check out this link
http://www.artofproblemsolving.com/Forum/resources.php?c=1&cid=16&sid=a6b7b3cc97f008d011ea73042624d3a2
In a mathematical competition 6 problems were posed to the
contestants. Each pair of problems was solved by more than 2
5 of the contestants.
Nobody solved all 6 problems. Show that there were at least 2
contestants who each solved exactly 5 problems.
check out this link
http://www.artofproblemsolving.com/Forum/resources.php?c=1&cid=16&sid=a6b7b3cc97f008d011ea73042624d3a2