Derek has his own company in which he sells bass fishing boats. He purchases GPS systems at $40,000 per system per year and installs them on the boat. I want to purchase a building which will cost $500,000 and build my own GPS systems. It will cost $30,000 per system per year. After 15 years, I will sell the building for $100,000 and retire. What is the minimum annual number of GPS systems are required to make it economically feasible? The interest rate is 10%. Show work and givens.