"paul and paula are the only members of society. the table above gives their marginal benefits from missile gunboats, a public good. determine the marginal benefit to society of the second missile gunboat."

The marginal benefir on the second boat will definitely increase.
The marginal benefit will tend to rise until it hit a certain amount where any addition to the same product will just seem abundant.
The point where the marginal benefit started to fall almost never exist on the secon purchase.