Unpaid invoices. The controller's office of a corporation is concerned that invoices that remain unpaid after 30 days are damaging relations with vendors. To assess the magnitude of the problem, a manager searches payment records for invoices that arrived in the past 10 months. The average number of invoices is 2875 per month, with relatively little month-to-month variation. Of all these invoices, 960 remained unpaid after 30 days.
(a) What is the total number of invoices studied? What is 
?
(b) Give the center line and control limits for a p chart on which to plot the future monthly proportions of unpaid invoices.