If the petition accompanying the proposed ordinance be signed by electors equal in number to fifteen (15) percent of the entire vote cast for all candidates for mayor at the last preceding general municipal election at which a mayor was elected, and contains a request that said ordinance be forthwith submitted to the vote of the people at the special election, then the mayor and council shall either:
(a) Pass said ordinance, without alteration, within twenty (20) days after the attachment of the clerk's certificate to the sufficiency of the accompanying petition, (subject to a referendary vote, under the provisions of chapter XX of this Charter); or
(b) Within twenty-five (25) days after the clerk shall have attached to the petition accompanying such ordinance his certificate of sufficiency, the mayor and council shall proceed to call a special election, at which said ordinance without alteration shall be submitted to the vote of the people.