CREATE TABLE table_name_64 (year INT, result VARCHAR, world_rank VARCHAR)

What Year has a Result smaller than 20.31, and a World Rank of 5th?
SELECT AVG(year) FROM table_name_64 WHERE result < 20.31 AND world_rank = "5th"
SELECT AVG("year") FROM "table_name_64" WHERE "5th" = "world_rank" AND "result" < 20.31