Periodic Inventory by Three Methods Oymac Appliances uses the periodic inventory system. Details regarding the inventory of applances at January 1 , purchases imvoices during the next 12 months, and the inventory count at December 31 are summarized as follows: 2. Deterinine the cost of the imventery en December 31 by the lastein, firstiout method If the inventory of a particular model tomprses one entire purchase plus a portion of another purchase acquired at a diferent unit cost, ine a separate line for each Durchase. If unts are in inventory at two different costs, enter the OLDEST units first. 4a. Which inventory method would be preferred for income tax purposes in periods of rising prices? 4b. Which inventory method would be preferred for income tax purposes in periods of declining prices