How many individuals watched the show that had a bbc ranking of 6?
CREATE TABLE "table1_24399615_10" (
  "episode_no" real,
  "airdate" text,
  "viewers" text,
  "bbc_three_weekly_ranking" text,
  "cable_rank" text
);
SELECT COUNT("viewers") FROM "table1_24399615_10" WHERE "bbc_three_weekly_ranking"='6';