Yikes! This problem isn't as simple as I thought. I already spent a couple of hours still couldn't get through. However, I found an example in following website, and I assume it might be helpful to solve it. If u r able to solve this problem please post the steps here. And I think 4.11% is the right answer because when u plug into a right online calculator you can get that answer.
http://mathforum.org/dr.math/faq/faq.interest.html