An automotive repair and maintenance shop uses 180 liters of coolant fluid per week, placing an order to re-stocking the coolant costs $10 and the cost of holding one liter in inventory for a year is $2. Assume that: the coolant only gets re-stocked when the inventory is empty, the lead time to receive the order once it's placed is negligible and that a year has 50 weeks. If the supplier only ships containers of 20 liters of fluid, how many containers should be ordered every time the inventory is re-stocked to minimize the total annual cost? Your answer should be an integer number