I do not understand periodic inventory by three methods. please help

Periodic Inventory by Three Methods Dymac Appliances uses the periodic inventory system. Detalls regarding the inventory of appliances at January 1 , purchases invoices during the next 12 months, and the inventory count at December 31 are summarized as follows: 1. Determine the cost of the inventory on December.31 by the firitih, nint-out method. If the inventory of a particular model comprises one entire purchase plus a portion of another purchase acpuired at a different unit cost, use a separate line for each putchaus. If units are in ifventery at two different costs, enter the unis PUACHASED MOST RECENTLY, RIAt. 1. Determine the cost of the inventory on December 31 by the first-in, first-out method. If the inventory of a particular model comprises one entire purchase plus a portion of another purchase acquired at a different unit cost, use a separate line for each purchase. If units are in inventory at two different costs, enter the units PURCHASED MOST RECENTLY first. 2. Determine the cost of the inventory on December 31 by the last-in, first-out method. If the Inventory of a particular model comprises one entire purchase plus a pertion of another purchase acquired at a different unit cost, use o separate line for each purchase. If units are in inventory at two different costs. enter tha OLDEST units first. 3. Determine the cost of the inventory on December 31 by the weighted average cost method. 4. would be prejerred for income tax purposes in periods of rising prices