Average Salary

Three friends - A, B an C want to find out what their average salary is without disclosing their own salaries to the others. How can they do it using only verbal communication?


A tells B some number, then B adds his salary to it and tells the result to C, then C adds his salary and tells the result to A. Now A subtracts the number he told B in the beginning, adds his own salary and divides by 3. Repeat the same procedure with B and C starting first.