It is assumed that there is a building of 100 square meters with a door & window area of about 30 square meters. The heat protection performance of high-performance doors & windows is better than that of ordinary doors & windows. Assumptions 1: If turn on the air conditioner, the power consumption of using ordinary doors & windows is 30 kWh/day; while the power consumption of using high-performance doors & windows is 25 kWh/day. Assumptions 2: The electricity price is 0.1 $/kWh. Assumptions 3: The investment cost of ordinary doors & windows and high-performance doors & windows is 200 $/m2 and 150 $/m2, respectively. Questions: How much is the cost saved by using high-performance doors and windows for each year? What is the payback period of investing high- performance doors & windows?