GreenTech Systems uses a periodic inventory system. It entered into the following transactions for May.

Date	Activities	Units Acquired at Cost	Units Sold at Retail
May 1	Beginning inventory	240 units @ $70 per unit
May 4	Purchase	450 units @ $75 per unit
May 11	Sales		400 units @ $105 per unit
May 20	Purchase	300 units @ $80 per unit
May 28	Sales		250 units @ $110 per unit

For specific identification, the May 11 sale consisted of 80 units from beginning inventory and 320 units from the May 4 purchase; the May 28 sale consisted of 150 units from the May 20 purchase and 100 units from the May 4 purchase.

Required:

1. Calculate the cost of ending inventory and COGS using FIFO and LIFO methods.
2. Determine the gross profit using the weighted average cost method.
3. Assess the impact of inventory costing methods on GreenTech Systems' profitability and tax liability.
4. Provide a recommendation on which inventory costing method should be adopted by GreenTech Systems, supported by financial analysis.