What is the rank of the athletes that have Notes of fb, and a Time of 6:47.30?
CREATE TABLE "semifinal_a_b_2" (
  "rank" real,
  "athlete" text,
  "country" text,
  "time" text,
  "notes" text
);
SELECT SUM("rank") FROM "semifinal_a_b_2" WHERE "notes"='fb' AND "time"='6:47.30';