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Mean Waiting Times of Customers at DMV

The following are mean waiting times and ranges for customers at local DMV cashier line measured at four different times of the day over the last three days. The times are given in minutes from the time a customer enters the cashier line, until the customer is called to the window.

X1-bar =20, R1 = 4; X2-bar = 24, R2 = 8; X3-bar = 34, R3 = 12
a Compute the Upper control limit for X- bar cahart
b Compute the lower Control limit for X-bar chart
c Compute the upper Control limit fot an R-chat
d Compute the lower control limit for an R-chat
e Plot the X-bar chart and determine if the process is out of control