CREATE TABLE table_3639 ("No. in series" FLOAT, "No. in season" FLOAT, "Title" TEXT, "Directed by" TEXT, "Written by" TEXT, "Original air date" TEXT, "U.S. viewers  ( in millions ) " TEXT)

How many people wrote the episode with 3.40 million U.S viewers?
SELECT COUNT("Written by") FROM table_3639 WHERE "U.S. viewers (in millions)" = '3.40'
SELECT COUNT("Written by") FROM "table_3639" WHERE "U.S. viewers (in millions)" = '3.40'