What is the highest number of viewers for the episode that has an 18-49 of 1.8/6?
CREATE TABLE "season_2_2009_2010_weekly_ratings" (
  "episode" text,
  "air_date" text,
  "timeslot" text,
  "18_49" text,
  "viewers" real
);
SELECT MAX("viewers") FROM "season_2_2009_2010_weekly_ratings" WHERE "18_49"='1.8/6';