What is the average year that has a car that won 0 stages with a position of DNF?
CREATE TABLE "dakar_rally_results" (
  "year" real,
  "class" text,
  "vehicle" text,
  "position" text,
  "stages_won" text
);
SELECT AVG("year") FROM "dakar_rally_results" WHERE "class"='car' AND "stages_won"='0' AND "position"='dnf';