the marginal social benefit from the production of the last unit of a good is $4,800. if the willingness to pay for that unit is $3,900, what is the external benefit from its production?

Petra contracted to paint bret’s house for $2,000. after beginning the job, petra realizes that the house is really quite big, and she’s not going to make enough profit, so she tells bret she wants another $500 to finish the job. bret doesn’t want to pay more, but he’s afraid that if she walks off the job, he’ll have trouble finding someone else to finish it, so he agrees. is bret legally obligated to pay the extra $500?