Two pumps can be used for pumping a corrosive liquid. A pump with a brass impeller costs

` 40,000 and is expected to last for three years. A pump with a stainless steel impeller will cost

` 95,000 and lasts for five years. An overhaul costing ` 15,000 will be required after 2,000 operating hours of the brass option, while an overhaul of ` 35,000 is estimated for the stainless steel option after 9,000 hours. If the operating cost of each pump is ` 25/hour, how many hours/year must the pump be required to justify the purchase of an expensive pump? (Use interest rate of 10 per cent/year.)