Inventory Management : Please answer bolded question and show work!

The corner store carries milk (in cartons) in their refrigerator. The store can place an order every week (i.e., 7 days). It takes about 3 days for the order to be delivered. Every month, the store is able to sell about 120 cartons of milk. The store is open every day (assume 30 days per month). If the inventory in the store is 25 cartons. 20. What is the order quantity? (Hint: Use periodic review model)