CREATE TABLE table_51831 ("Year" FLOAT, "Competition" TEXT, "Venue" TEXT, "Position" TEXT, "Event" TEXT, "Notes" TEXT)

What year was the Competition of World Junior Championships with a 20th (qf) position?
SELECT AVG("Year") FROM table_51831 WHERE "Competition" = 'world junior championships' AND "Position" = '20th (qf)'
SELECT AVG("Year") FROM "table_51831" WHERE "Competition" = 'world junior championships' AND "Position" = '20th (qf)'