What episode number in the series was viewed by 13.66 million people in the U.S.?
CREATE TABLE "table1_25668962_1" (
  "no_in_series" real,
  "title" text,
  "directed_by" text,
  "written_by" text,
  "original_air_date" text,
  "production_code" text,
  "u_s_viewers_million" text
);
SELECT MAX("no_in_series") FROM "table1_25668962_1" WHERE "u_s_viewers_million"='13.66';