The bonds shall be payable serially within 20 years from their date, as follows:
(a) Not less than 10 percent of the aggregate face value of bonds issued shall be payable within 10 years from their date.
(b) Not less than 10 percent of the aggregate face value of bonds remaining unpaid at the end of 10 years shall be payable each year beginning with the eleventh (11th) year from their date, until the whole amount of the bonds is paid.