lifo method and fifo

The following information relates to the receipts and issues of materials at S Co for the month of August. Units received Units issued 3 August 400 @ $17 each 5 August 250 @ $18 each 12 August 120 18 August 480 25 August 275 @ $16 What is the value of the units issued on 18 August using the LIFO method of inventory valuation? and also find the closing inventory using FIFO method IN FIFO METHOD in this i get confused in which they subtract 120 FROM 400 AND GET 280 WITH PER UNIT COST OF 17 AND THEN TOGETHER THEY FIND FOR AUG 18 ISSUES BY 280 AND 200 AND MULTIPLYING IT WITH UNIT COSTS IN LIFO VALUATION IN THIS THEY TAKE 250 AND SUBTRACT IT FROM 120 TO GET 130 AND THEN TAKE 130 AND SUBTRACT 480 TO GET 350 CAN U EXPLAIN ME FULLY THE SUM WHAT THEY DO I COULD'NT GET WHAT THEY DO IN IT