Each issue or series of bonds must bear interest at an effective rate over the life of the bonds not to exceed 11 percent a year or that rate of interest that is 110 percent of the rate of the Bond Buyer Index of 20 Municipal Bond Average Yields for the week previous to the date of sale of the bonds, whichever is higher. The bonds must mature in not more than 30 years from date of issue, unless a longer period is specifically authorized by statute.