The number of defective microchips produced by machine # 6 during each of the last 24 hours of operation is shown in the frequency table below:

Defectives x                No. of hours f(x)

0………………………….     9

1…………………………      6

2…………………………      4

3…………………………      3

4…………………………      2

                  Total = 24

a. Compute the mean number of defectives per hour over the 24-hour period. (Treat the data as a population.) 

b. What is the median number of defectives per hour? 

c. Compute the variance and the standard deviation for the hourly defective counts.