Answer:
More than $40, as the marginal benefit will exceed the marginal cost.
Explanation:
A firm should continue to produce a good or service, to hire an additional worker, and to engage in any type of economic activity as long as the marginal benefit is higher than the marginal cost.
In this case, operating one room for the night costs $40. That is the marginal cost. What the customer pays for renting the room for the night is the marginal cost. Therefore, as long as the customer pays more than $40 for the night, the hotel should rent out the room, because the marginal benefit is higher than the marginal cost.