CREATE TABLE table_204_939 (id DECIMAL, "year" DECIMAL, "chassis" TEXT, "engine" TEXT, "start" TEXT, "finish" TEXT)

what is the average engine type listed ?
SELECT "engine" FROM table_204_939 GROUP BY "engine" ORDER BY COUNT(*) DESC LIMIT 1
SELECT "engine" FROM "table_204_939" GROUP BY "engine" ORDER BY COUNT(*) DESC NULLS LAST LIMIT 1