The following table shows the weekly working hours of some employees (including full-time and part-time) of a company:

Working hours (hours) / Group boundary/ class mark/ frequency/ Hours less than/ cumulative frequency

21 - 25 10

26 - 30 12

31 - 35 16

36 - 40 32

41 - 45 22

46 - 50 8

Total: 100

(a) Using the data in the frequency distribution table, find the average number of hours worked per week and the sample standard deviation.

(Both can be obtained directly by computer) (4 points)

(b) Find the median and mode of the above grouped data. (Calculation by formula is required) (8 points)