a) The president of a retail store annually checks the number of sales each of his employees make in a month. The information from a sample of 100 employees is summarized in the following table. Number of Sales in a month Number of Employees 0-under 10 10 10-under 20 25 20-under 30 30 30-under 40 20 40-under 50 15 What is the approximate mean number of sales