The governing authority issuing the bonds shall, by resolution, fix the time within the maximum period prescribed, when the bonds shall become payable.  The bonds shall be payable in annual installments and the amount payable in each year shall be so fixed that, when the annual interest is added to the principal amount to be paid, the total amount payable in each year shall be as nearly equal as practicable.  It shall be an immaterial variance if the difference between the largest and smallest amounts of principal and interest, payable annually during the term of the bonds, does not exceed five per cent of the total authorized issue.