A manufacturing plant is considering the purchase of a new filter press can be purchased for $30,000 and a salvage value of $16,000. The operating costs in the 1st year are $5,000. For the remaining years these costs increase each year by $2,000 over the previous year's operating costs. The filter press has a maximum life of 8 years without any maior repair. The firm's MARR is 15%. What is the optimum replacement interval for this filter press