Company B is considering purchasing an equipment. The initial investment is $500,000 and annual revenues are expected to be $200,000 over the 8-year life of the equipment. Annual expenses are $100,000 at the end of year one and will increase by $10,000 each year thereafter. The resale value of the equipment is $20,000. What is the discounted payback period of the equipment, if the interest rate is 10% per year?