Should the result of such election show that it has carried in favor of the issuance of bonds by a majority vote in each district or in the one district, as the case may be, then it shall be the duty of the board of supervisors to issue the bonds of such districts or district affected thereby, as the case may be, for the purpose aforesaid in any amount which, with other outstanding district road or bridge bonds, will not exceed fifteen percent (15%) of the assessed valuation of said districts or district affected thereby, as the case may be.