What is the average Density of 53 No. munic.?
CREATE TABLE "list" (
  "abbr" text,
  "canton" text,
  "capital" text,
  "density_per_km" real,
  "no_munic" real,
  "s_official_language" text
);
SELECT AVG("density_per_km") FROM "list" WHERE "no_munic"=53;