Final answer:
The average inventory of Heartgard boxes, calculated by adding the beginning and ending inventory and dividing by 2, is 25 boxes.
Step-by-step explanation:
To calculate the average inventory of Heartgard boxes, add the number of boxes on January 1 to the number on December 31 and divide by 2.
With 20 boxes on January 1 and 30 on December 31, the average inventory formula is (20 + 30) / 2. Therefore, the average inventory is 25 boxes.