What is the mean number of laps when the grid is less than 19 and time/retired is +21.3 secs?
CREATE TABLE "race" (
  "driver" text,
  "team" text,
  "laps" real,
  "time_retired" text,
  "grid" real
);
SELECT AVG("laps") FROM "race" WHERE "grid"<19 AND "time_retired"='+21.3 secs';