A granary purchases a conveyor used in the manufacture of grain for transporting, filling, or emptying. It is purchased and installed for $72,000 with a market value for salvage purposes that decreases at a rate of 20% per year with a minimum of value $2,350. Operation and maintenance is expected to cost $14,400 in the first year, increasing $1,200 per year thereafter. The granary uses a MARR of 15%. What is the optimum replacement interval for the conveyor? ___________ years.