CREATE TABLE table_27991 ("Series #" FLOAT, "Episode title" TEXT, "Directed by" TEXT, "Written by" TEXT, "Original air date" TEXT, "Prod. code" FLOAT)

Who wrote the episode with production code 5?
SELECT "Written by" FROM table_27991 WHERE "Prod. code" = '5'
SELECT "Written by" FROM "table_27991" WHERE "Prod. code" = '5'