4-36. A geothermal heat pump can save up to 80% of the annual heating and cooling bills of a certain home in the northeastern U.S. In this region, the yearly cost of conventionally heating and cooling a 2,000 square foot house is about $2,500. With a federal tax credit of 30% on the total installation of the system, how much can a homeowner afford to spend for a geothermal heat pump if the interest rate is 9% per year? The expected life of the heat pump is 15 years. (4.7)