Which Capacity is the lowest one that has a Rank smaller than 15, and a City of belgrade?
CREATE TABLE "davis_cup_and_federation_cup_venues" (
  "rank" real,
  "stadium" text,
  "capacity" real,
  "city" text,
  "country" text
);
SELECT MIN("capacity") FROM "davis_cup_and_federation_cup_venues" WHERE "rank"<15 AND "city"='belgrade';