Scenario 10-4 the Demand Curve for Fire Extinguishers Slopes Downward and the and the Supply

Scenario 10-4
The demand curve for fire extinguishers slopes downward and the supply curve for fire extinguishers slopes upward. The production of the 500^th fire extinguisher entails the following:
• a private cost of $10;
• an external cost of $0;
• a private value of $9;
• an external benefit of $3.
-Refer to Scenario 10-4. In order to maximize the total benefit of fire extinguishers to society as a whole, should the number of fire extinguishers produced be less than, equal to, or greater than 500? Explain.