The annual demand for a notebook binder at Duncan’s Stationary Shop is 9,000 units. The owner operates her business 300 days a year and reviews the inventory once every 5 days. It takes 4 days to receive a delivery from the supplier of the notebook binder once an order is placed. The standard deviation of the daily demand is 4. How many notebook binders should the owner order each time? Please provide the formula, at least one step of calculation, and the correct answer for full credit.