This isn't homework, I've got a solution, however it doesn't protect against cheaters.

Three coworkers would like to know their average salary. However, they are self-conscious and don't want to tell each other their own salaries, for fear of either being ridiculed or getting their houses robbed. How can they find their average salary, without disclosing their own salaries?

Now, a solution that requires the last person to tell the group the sum isn't allowed because that person could cheat.

Solution: http://karavi.wordpress.com/2009/12/18/solutions-to-wu%E2%80%99s-puzzles-and-riddles-ghetto-encryption-2-medium/