At the expiration of said period of 30 days the City Treasurer shall assemble all of said copies in his office as one instrument, and shall examine the same and ascertain and certify thereon whether the signatures thereto amount to at least 15% of the number of persons voting at the last preceding municipal election. If such signatures do amount to such percent, he shall at once serve notice of that fact upon the City Commissioner or City Commissioners designated in the petition, and also deliver to the election authorities a copy of the original petition with his certificate as to the percentage of registered voters who signed the same, and a certificate as to the date of his last mentioned notice to the City Commissioner or City Commissioners designated in the petition.