John Davis, a recent IE graduate from Tennessee Technological University, bought an SUV for $30,000 with a down payment of $10,000. John had a little business on the side and did not have a girlfriend when he was at school and hence he was able to save the $10,000 for his dream car. He expects to take good care of the car and the dealership, owned by John's uncle, agrees to take the car back for $8,000 at the end of 4 years. What is the effective interest rate?