Hello.. Yesterday, we took this business administration exam. The first question in the exam was about Daud tools. In the question, we were supposed to find out the marginal cost of investment in account receivables (part b of the question). Has anyone done it correctly or verified the answer?

P.S. I know one doesn’t need to fidget about things one doesn’t have any control on but I’ll really value any response that could help me get rid of my apprehension regarding the correctness of the answer.