A factory supervisor is concerned that the time it takes workers to complete an important production task (measured in seconds) is too erratic and adversely affects expected profits. The supervisor proceeds by randomly sampling 5 individuals per hour for a period of 10 hours. The sample mean and range for each hour are listed below.

Hour X R

1 18.4 25

2 16.9 27

3 23.0 30

4 21.2 23

5 21.0 24

6 24.0 25

7 19.3 12

8 15.8 14

9 20.0 13

10 23.0 11

Referring to the problem statement, suppose the supervisor constructs an X-bar chart to see if the process is in-control. What is the center line of the chart?