You should be able to solve this.

A group of jealous professors is locked up in
a room. There is nothing else in the room but pencils
and one tiny scrap of paper per person. The profes-
sors want to determine their average (mean, not me-
dian) salary so that each one can gloat or grieve over
his or her personal situation compared to their peers.
However, they are secretive people, and do not want to
give away any personal salary information to anyone
else. Can they determine the average salary in such a
way that no professor can discover any fact about the
salary of anyone but herself? For example, even facts
such as "three people earn more than $40,000" or "no
one earns more than $90,000" are not allowed.

>>7
They add all of their numbers together (mod m, right?), and they will get the sum of their salary mod m

Assuming there's an eraser at the end of the pencils, there's a simpler, though less secure method: everybody writes down a random number and passes the paper to the person next to them.  The next person adds his salary to the number, and erases the original number.  This continues until the paper returns to the original profs.  They add their own salary to the resulting number, subtract the original random number, divide by the # of profs in the room, and we're done.