Answer:
 Concept/Method       W-A          FIFO        LIFO
Ending Inventory   $  14,280     10,240       9,300
COGS                     $  21,420     25,460    26,400
Explanation:
First, we calcualte the goods available for sale through the year:
 Jan. 1    Inventory 30 units at $110   =   3,300
 Mar. 10  Purchase 60 units at $120 =    7,200
 Aug. 30  Purchase 10 units at $124 =  12,400
 Dec. 12 Purchase<u> 100 units</u> at $128 =  <u>12,800</u>
Goods Available:  200 units  Cost:       35,700
<u>Weighted average:</u>
we divide the cost of goods available over the units :
35,700 / 200 = 178.5
Then we multiply for COGS and EI
120 units x  $ 178.5 = $  21,420 COGS
  80 units x $ 178.5 = $   14,280 Ending Inventory
<u>FIFO</u>
The first untis(oldest) are COGS and the last are inventory.
we determinate the ending inventory from the last row:
 Dec. 12 Purchase<u> 100 units</u> at $128 =  <u>12,800</u>
On ending inventory there is 80 units so:
80 units x 128 = 10,240
Then COGS will be the diffrence between cost of good available and ending inventory:
35,700 - 10,240 = 25,460
<u>LIFO</u>
the last units (newest) are COGS and the first are inventory
we determinate the ending inventory from the first row:
 Jan. 1    Inventory <u>30 units </u>at $110   =   3,300
Ending invenotory 80 units - 30 units = 50 more units
we "grab" one more row:
 Mar. 10  Purchase 60 units at $120 =    7,200
from we he need 50 units at 120
so ending inventory is:
30 units at $  110   =  3,300
50 units at $ 120    = 6,000
         Total                   9,300
Then, COGS is calculated by dfference like FIFO:
35,700 availalbe goods - 9,300 ending inventory = 26,400