what is the rating where the rank is 48?
CREATE TABLE "u_s_nielsen_ratings" (
  "num" real,
  "episode" text,
  "rating" text,
  "share" real,
  "rating_share_18_49" text,
  "viewers_millions" text,
  "rank_timeslot" text,
  "rank_night" text,
  "rank_week" text
);
SELECT "rating_share_18_49" FROM "u_s_nielsen_ratings" WHERE "rank_week"='48';