As I read the puzzler, there are only 7 employees. If all 7 employees made the same salary, the average would be obvious to the employees. However, it was stated that the salaries are all different. Now if one knows the highest salary and the lowest salary, one would know the median salary. The puzzler, though, calls for one to know the average salary which I presume is the mean salary. This tells me that the median salary and the mean salary must be equal.
The employees then question each other by one asking the question “How many earn above a certain amount?” Each employee writes “Yes” or “No” on a piece of paper without divulging his name. The number of people above the amount is recorded. The question is then repeated with a higher amount. The amount is increased in equal intervals. This continues until there are three “Yes” votes and four “No” votes. The averaage salary is then the amount between the amount that generated four “Yes” votes and the amount that generated three “Yes” votes. One can continue selecting amounts between the one that elicited four “Yes” votes and three “Yes” votes to pin down the average value. In this way, nobody divulges his salary.