Answer:
There is a positive externality worth $3.
Explanation:
Quantity 4 5 6 7
Private marginal benefit ($) 6 4 2 0
Social marginal benefit ($) 9 7 5 3
The externality is found by calculating the difference between social benefit and private benefit.
Here, we see that each level of output, the social benefit is greater than the private benefit. So we can say that a positive externality exists in the market.
The externality is equal to
When quantity=4,
$(9-6)=$3
When quantity=5,
$(7-4)=$3
When quantity=4,
$(5-2)=$3
When quantity=4,
$(3-0)=$3
So, there is an externality worth $3.