Whenever the petition shall be found to be sufficient, the city clerk Shall so certify to the council at its next meeting, stating the number of petitioners, and the percentage of the total number of voters which they constitute, and the council shall at once read the measure and refer it to an appropriate committee, which may be a committee of the whole. The committee or council shall thereupon provide for public hearings upon the measure, after the holding of which the measure shall be finally acted upon by the council not later than sixty-five days after the date upon which such measure was submitted to the council by the city clerk. If the council shall fail to pass the proposed measure, or shall pass it in a form different from that set forth in the petition and is unsatisfactory to the petitioners, the proposed measure shall be submitted by the council to the vote of the electors at the next regular municipal election. But in case the number of signers of said petition is equal to at least fifteen percent of the total number of voters voting at the last regular municipal election, then the council shall call a special election upon the measure to be held not less than thirty nor more than forty-five days from such date, unless a regular election is to occur within three months, in which case it may be submitted at such regular municipal election. In case the council passes the proposed measure with amendments and at least four-fifths of the committee of petitioners do not express their dissatisfaction with such amended form by a certificate filed with the city clerk within ten days from the passage thereof by the council, then the measure need not be submitted to the electors.