Date Activities Units Acquired at Cost Units Sold at Retail
Jan. 1 Beginning inventory 5 units @ $500/unit
Jan. 5 Purchase 20 units @ $550/unit
Jan. 9 Sales 21 units @ $850/unit
Jan. 18 Purchase 6 units @ $600/unit
Jan. 25 Purchase 10 units @ $620/unit
Jan. 29 Sales 8 units @ $950/unit
Totals 41 units
29 units

For specific identification, the January 9 sale consisted of 4 units from beginning inventory and 17 units from the January 5 purchase; the January 29 sale consisted of 2 units from the January 18 purchase and 6 units from the January 25 purchase

Your Cost of Goods Sold is:
4 units of Jan 1 inventory @$500 a unit
17 units from Jan 5 purchase @$550 a unit
2 units from Jan 18 purchase @$600 a unit
6 units from Jan 25 purchase @ $620 a unit

Now you just need to calculate the total cost for each sale and add the totals together to get Cost of Goods Sold.

To get ending inventory you need to calculate the costs of your beginning inventory plus all your purchases minus your Cost of Goods Sold (calculated above).