Final answer:
The provided question about the sitting fee per subject does not include enough context to determine a specific answer. If it were related to educational fees such as a tutoring service, an example equation could be made based on the given information.
Step-by-step explanation:
The question you asked does not provide enough context or information on what the sitting fee per subject actually refers to. The values provided ($10, $20, $30, $40) could be fees associated with anything from educational services to exams. However, without specific context, it is not possible to accurately determine what the correct sitting fee might be.
In terms of the reference information provided, which seems to discuss interest rates and bond pricing or a tutoring school's enrollment fee equation, these do not give a clear indication of what a sitting fee per subject would be either.
If the sitting fee is related to a tutoring school or educational service, and an example would be needed, we could potentially formulate a scenario or an equation. For instance, if a tutoring school charges a one-time enrollment fee and annual tuition, an equation could be as follows: Total Cost = Enrollment Fee + (Tuition per year × Number of years).