Joe, Henry, and Noela, after graduating together, started a business with a $28 000 investment from their parents. For the first two years, they were charged interest at 6% compounded monthly, but no payments were required during that period. For the next five years, they would make payments of $625 at the beginning of every month, and the interest rate changed to semi-annual compounding. What is the nominal interest rate they were charged during the last five years?