Following the election the board of supervisors of elections shall promptly make a return to the president of the council of the number of votes cast for the amendment and of the number of votes cast against the amendment. If, at such election, the majority of the votes cast for and against the amendment shall be in favor thereof, the amendment shall be adopted and become a part of the charter from and after the thirtieth day after such election. Promptly after the return of the votes, the president of the council shall give public notice of the result thereof in such manner as he shall determine, and if the proposed amendment is adopted, of the effective date thereof. (Mont. Co. Code 1965, § 2-5; 1968 L.M.C., Ex. Sess., ch. 8, § 5.)