Marginal utility is negative only when quantity demanded is negative

In a hypothetical country, the average wage of five 40-year-old citizens with college education is $36,896, and the average wage of five 40-year-old citizens with high school education is $25,864

What is the returns-to-college education in the country? Is there any limitation of this analysis? Explain your answer.

Black Oil Company considered building a service station in a new location. The owners and their accountants decided that this was the profitable thing to do. However, soon after they made this decision, both the interest rate and the cost of building the station changed. In which case do these changes both make it less likely that they will now build the station?

a. Interest rates rise and the cost of building the station rises. b. Interest rates rise and the cost of building the station falls. c. Interest rates fall and the cost of building the station rises. d. Interest rates fall and the cost of building the station falls.