To calculate the cost of goods sold (COGS), we first determine the total value of the inventory sold during the period. We start with the beginning inventory of $9,000. Then, we add the value of the additional inventory purchased, which is calculated by multiplying the cost of each item by the number of units purchased. For puzzles, the cost is $15.00 per unit for 10 units, totaling $150.00. For shirts, the cost is $18.00 per unit for 15 units, totaling $270.00. For key chains, the cost is $5.50 per unit for 20 units, totaling $110.00. Adding these together gives us a total additional inventory value of $530.00. Next, we subtract the ending inventory value of $1,000 from the sum of the beginning inventory and additional inventory. The formula is: COGS = Beginning Inventory + Additional Inventory - Ending Inventory. Plugging in the numbers: COGS = $9,000 + $530.00 - $1,000, which simplifies to COGS = $8,530.00. Therefore, the cost of goods sold over this period is $7,530.00.