Answer:
Yes their combined contribution of $120 is sufficient to pay the security guard
Explanation:
This is a form of non-rival service that does not get reduced by consumption by one of the parties. The security guard protects the entire neighbourhood.
Also it is a non-excludable service. If one party does not pay he will still enjoy the service.
Based on their income information and the fact that they are taxed 1%
Jack will pay 0.01 * 4,000 = $40
Jill will pay 0.01 * 8,000 = $80
The total sum is 40 + 80 = $120
As the security guard requires $120 to do the job, their contribution is sufficient.
However the value to Jack is $50 and he pays $40 so he has an excess value of $10
Jill gets a value of $90 but pays $80, so she gets an excess value of $10