FIFO inventory method is used.
Explanation
Health Defense is using FIFO (First In First Out Method) Inventory method. Under periodic inventory system, no frequent recording is made throughout the year. Rather physical count is done at the end of the period and cost is assigned to the remaining inventory accordingly. If FIFO inventory method is used in periodic inventory system, ending inventory is valued using the most recent purchases which is the case here.
Opening Inventory : 18 units
Purchased : 30 units
Sold : 24 units
Purchased : 24 units
Sold : 27 units
Ending Inventory therefore is : 18+30-24+24-27 = 21 units
Dividing the amount of ending inventory by the remaining units in inventory will give us $17 which is the price of most recent purchases.
$357/21 = $17