Count the average Wins which has a Class of 250cc, and a Year of 1972?
CREATE TABLE "grand_prix_motorcycle_racing_results" (
  "year" real,
  "class" text,
  "team" text,
  "points" real,
  "wins" real
);
SELECT AVG("wins") FROM "grand_prix_motorcycle_racing_results" WHERE "class"='250cc' AND "year"=1972;