25 The numbers of courses taught per semester by a random sample of university professors are shown in the histogram. Make a frequency distribution for the data. Then use the table to estimate the sample mean and the sample standard deviation of the data set. 20 21 18 20- 15 Number of professors 10 2 4 5 Number of courses taught per semester X t 1 4 2 20 3 21 4 18 The sample mean is - (Round to one decimal place as needed.) The sample standard deviation is s- (Round to one decimal place as needed.)