Answer:
The total surplus is $30.
Step-by-step explanation:
The total surplus before the tax is the sum of the producer surplus, and the consumer surplus.
In this case, the producer is David, who is walking the dog. David is making $50 per week for the task, while his opportunity cost (the value of his time if he did not walk the dog and did something else instead) is $30. As a result, his surplus, the producer surplus, is $20.
Carolyn, the consumer, is paying David $50, but she places a value of $60 on this activity, as a result, her surplus, the consumer surplus, is $10.
The sum of producer surplus ($20), and consumer surplus ($10), is $30.