11-13. A group of private investors borrowed $30 million to build 300 new luxury apartments near a large university. The money was borrowed at 6% annual interest, and the loan is to be repaid in equal annual amounts over a 40-year period. Annual operating, maintenance, and insurance expenses are estimated to be $4,000 per apartment. This expense will be incurred even if an apartment is vacant. The rental fee for each apartment will be $12,000 per year, and the worst-case occupancy rate is projected to be 80%. Investigate the sensitivity of annual profit (or loss) to (a) changes in the occupancy rate and (b) changes in the annual rental fee. (11.3)