What is the average number of borough councilors from Brompton?
CREATE TABLE "list_of_sherbrooke_boroughs" (
  "borough" text,
  "components" text,
  "population" real,
  "number_of_borough_councilors" real,
  "number_of_municipal_councilors" real
);
SELECT AVG("number_of_borough_councilors") FROM "list_of_sherbrooke_boroughs" WHERE "borough"='brompton';