Such amendment adopted by City Council shall become effective 30 days after the date of such adoption, unless within 30 days after the passage of the ordinance there is presented to the City Clerk a petition, signed by a number of qualified voters residing in the city, equal to but not less than 10% of the total vote cast in such areas at the last preceding general election at which a Governor was elected, requesting the City Council to submit the Zoning Amendment to the electors of the city for approval or rejection at the next general election.