How do you do this problem
Mercedes Brown starts a merchandising business on December 1 and enters into three inventory purchases:

December 7 10 units @ $ 9 cost
December 14 20 units @ $ 10 cost
December 21 15 units @ $ 12 cost

Brown sells 18 units for $35 each on December 15. Seven of the sold units are from the December 7 purchase and eleven are from the December 14 purchase. Brown uses a periodic inventory system. Determine the costs assigned to the December 31 ending inventory inventory when costs are assigned based on (a) FIFO, (b) LIFO, (c) weighted average, and (d ) specific identification. (Round per unit costs to 3 decimals places. Round your answers to the nearest dollar amount. Omit the "$" sign in your response.)

Ending Inventory
a. FIFO $
b. LIFO $
c. Weighted Average $
d. Specific Identification $

Here is an online text showing how to do inventory. Please make an attempt to do the problem, and then show your attempts and someone can guide you from there:
Principles of Accounting Chapter 8 (http://www.principlesofaccounting.com/chapter%208.htm#inventory%20costing%20methods)