A Quick Post supervisor is looking for ways to reduce stress in the mail-sorting department. With the existing arrangement, stamped letters are machine-cancelled and loaded into tubs with 375 letters per tub. The tubs are then pushed to postal clerks, who read and key postal codes into an automated sorting machine at the rate of 1 tub per 375 seconds. To overcome the stress caused when the stamp cancelling machine outpaces the sorting clerks, a pull system is proposed. When the clerks are ready to process another tub of mail, they will pull the tub from the cancelling machine area. How many tubs should circulate between the sorting clerks and the cancelling machine if 90 000 letters are to be sorted during an eight-hour shift, the safety stock policy variable, a, is 0.18, and the average waiting time plus materials handling time is 25 minutes per tub?