Tell me the average wins for class of 50cc and rank of 8th
CREATE TABLE "motorcycle_grand_prix_results" (
  "year" real,
  "class" text,
  "team" text,
  "points" real,
  "rank" text,
  "wins" real
);
SELECT AVG("wins") FROM "motorcycle_grand_prix_results" WHERE "class"='50cc' AND "rank"='8th';