Answer: $800 since the benefit of one more unit goes to both individuals.
Step-by-step explanation:
The benefit of one more public good will go to both Sam and Alex. For this reason, the total benefit is the sum of the benefits that go to the both of them.
Sam benefits $500 and Alex benefits $300.
= 300 + 500
= $800
$800 goes to the both of them and so is the social benefit of one more unit of the public good.