A U.S. Postal Service office is planning to adopt a pull system. With the current push system, a machine cancels the stamped letters, which are loaded into tubs with 400 letters in each tub. The tubs are then sent to the postal clerks for sorting. The clerks, with the help of an automatic sorting machine, read and key in the zip codes at the rate of 1 tub per 400 seconds. Nevertheless, every now and then, the stamp canceling machine outpaces the sorting operation, causing considerable stress on the clerks. Under the proposed pull system, employees will pull a tub of letters with canceled stamps only when the clerks are ready to sort another tub. Given a safety stock of 20% for the tubs and an average waiting and materials handling time of 20 minutes per tub, calculate the number of tubs that should be circulating between the sorting clerks and the machine canceling area if 100,000 letters are to be sorted during a 10-hour shift.