A state department of tourism and recreation collects data on the number of cars with out-of-state license plates in a state park. (The group's position is that more out-of-state plates means the state's advertising programs are working.) The sample size is fixed at n=90 each day. Data from the previous 20 days indicate the following number of out-of-state license plates:

Day Out-of-state Plates

1 24
2 33
3 18
4 21
5 26
6 31
7 15
8 18
9 23
10 13
11 22
12 15
13 25
14 33
15 16
16 22
17 34
18 10
19 23
20 13

a. Calculate the overall proportion of "tourists" (cars with out-of-state plates).

b. Calculate the LCL and UCL for these data.

c. Plot the control chart. Are all points within the control limits?

d. What action do you suggest for improving this process?