At the expiration of said period of thirty (30) days, the clerk of the city commission 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 fifteen percent (15%) of the registered voters in the municipality at the last regular municipal election of the city. If such signatures do amount to such percent, he shall at once serve notice of that fact upon the commissioner or 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 commissioner or commissioners designated in the petition.