The following text represents a valid contest question
in which I have erased one number :

Let S be a set of 20 distinct positive integers,
each less than EN(=THE ERASED NUMBER).
Show that there exist four distinct elements a, b, c, d , all in S,
such that a + b = c + d.

What maximal value of EN guarantees the existence of these elements ?

Thank you all.

I will post as an "official" solution, referring to your results.

Ady

Posted by Ady TZIDON on 2010-07-20 20:05:22