Final answer:
Donna's gain or loss is $20,000.
Step-by-step explanation:
To calculate Donna's gain or loss, we need to subtract her total expenses from the selling price of the house. Donna's total expenses include the cost of the house ($300,000), the cost of adding the new room ($10,000), and the sales commission ($20,000). So, her total expenses are $300,000 + $10,000 + $20,000 = $330,000.
The selling price of the house is $350,000. To find the gain or loss, subtract the total expenses from the selling price: $350,000 - $330,000 = $20,000 gain.