What happens to the total cost of a good the longer it takes you to pay it off? Why does this happen?

Inventory by three methods; cost of goods sold the units of an item available for sale during the year were as follows: jan. 1 inventory 20 units at $1,800 may 15 purchase 31 units at $1,950 aug. 7 purchase 13 units at $2,040 nov. 20 purchase 16 units at $2,100 there are 18 units of the item in the physical inventory at december 31. determine the cost of ending inventory and the cost of goods sold by three methods, presenting your answers in the following form: round your final answers to the nearest dollar. cost inventory method ending inventory cost of goods sold a. first-in, first-out method $ $ b. last-in, first-out method $ $ c. weighted average cost method $ $