CREATE TABLE table_390 ("Series #" FLOAT, "Title" TEXT, "Directed by" TEXT, "Written by" TEXT, "Musical guest and song" TEXT, "Original air date" TEXT, "Production code" FLOAT)

Who were the writers for series number 20?
SELECT "Written by" FROM table_390 WHERE "Series #" = '20'
SELECT "Written by" FROM "table_390" WHERE "Series #" = '20'