Question: Mark invests 1000 dollars. Two years later, he receives a payment from the investment of 2500 dollars. Two years after that, he invests another 412.4 dollars, and two years after that, he adds 2875 dollars more to the investment. Two years later, he receives a final payment of 1796.76 dollars that closes out the investment. What are the possible effective rates of interest? (If you find more than one, list them separated by commas.)