Microeconomics question about marginal product and marginal revenue product of labor?

Question

As the city population grows, crime increases. The city has the choice of hiring more officers, or installing security cameras. Here are the details:
- Each prevented crime is worth $1000 to the city.
~ Each police officer would be paid $20,000/year.
~ Each security camera would cost $8,000/year.
~ Number of CrimesPrevented Per Year-Number of Officers Hired:
21 - 1, 47 - 2, 76 - 3, 104 - 4, 128 - 5, 149 - 6, 165 - 7, 178 - 8, 188 - 9, 195 - 10.
~ Number of Crimes Prevented Per Year -Number of Camera Installations: 2 - 1, 9 - 2, 18 - 3, 26 - 4, 32 - 5.
Here are the questions:
~ What is the marginal product and marginal revenue product of labor for police officers? ~ What is the optimal number of officers to be hired if the city only hires police officers?
~ What is the marginal product and marginal revenue product for the cameras?
~ What is the optimal number of cameras if the city decides to only use cameras?

Answer 1

-$28,000
-Optimal number of officers=5
-negative $8,000
-Optimal number of cameras=5