A housing estate has 100 residents who are concerned about security. The accompanying table gives the total cost of hiring a 24-hour security service as well as each individual resident's total benefit. (a) Explain (in 50 words or less), using precise economic language, why the security service is a public good for residents of the estate. (b) If the residents act together, how many security guards will they hire? (use marginal analysis to justify your answer and show all working).
Quantity of security guards
Total Cost
Total individual benefit to each resident $0 10 16 18 19
0 1 2 3 4
$0 150 300 450 600