Final answer:
The optimal usage fee for farmers is $0.00.
Step-by-step explanation:
The optimal usage fee for farmers is $0.00. The optimal usage fee is calculated by setting the marginal cost equal to the rental charge. In this case, the marginal cost is $10.00 per minute, so setting it equal to the rental charge of $P, we get $10.00 = P. Therefore, the optimal usage fee for farmers is $0.00.