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.

The first one adds a random number (like 2450) to his salary, writes the sum down on the paper and passes it to the next person. The next one adds his salary and another random number, and so on.
Then the paper returns to the first guy and he'll substract his random number from the sum. The next guy substracts his random number. So the paper passes everyone two times.
At the end they have the sum of all salaries from which we can get the average.