Answer: Marginal revenue is -$500.
Explanation: The marginal revenue is calculated as the change in total revenue subtracted by the change in quantity.
Total revenue is calculated by multiplying the price by the quantity:
At a quantity of 20 driveways, the total revenue is = 20 × $10,000 = $200,000
At a quantity of 21 driveways, the total revenue is = 21 × $9,500 = $199,500
Marginal revenue = $199,500 - $200,000
= -$500