What is the average quantity for coaches manufactured in 1921/23, and had 40 seats?
CREATE TABLE "iron_coaches" (
  "class_to_1928" text,
  "class_from_1928" text,
  "seats" text,
  "quantity" real,
  "year_s_of_manufacture" text,
  "remarks" text
);
SELECT AVG("quantity") FROM "iron_coaches" WHERE "year_s_of_manufacture"='1921/23' AND "seats"='40';