An investor buys a rental property for $500,000. This property is expected to generate $2,000 rental income for the first year, $4,000 rental income for the second year and $6,000 rental income for every year after that. At the end of four years, the investors plan to sell the rental property, which is expected to sell at a gain of $100,000. What is the internal rate of return (IRR) for this investment?