Answer:
The marignal revenue that another worker would bring to Joe under these circumstances is $90.
If joe hires a new worker, the worker can repair 3 appliances per hour, and the mininum charge for appliance-repair is $30 plus parts. The marignal revenue is:
$30 x 3 appliances = $90
Because wages are equal to the marginal product of labor, the maximum amount that Joe would pay to a new person is $90.