E6-14 Inventory by three methods

The units of an item available for sale during the year were as follows:

Jan. 1	Inventory	54 units at $1,200
Mar. 6	Purchase	108 units at $1,380
Jul. 14	Purchase	126 units at $1,560
Nov. 2	Purchase	112 units at $1,650

There are 90 units of the item in the physical inventory at December 31. The periodic inventory system is used. Determine the inventory cost by (a) the first-in, first-out method, (b) the last-in, first-out method, and (c) the average cost method.

Please explain this to me as detailed as possible.