CREATE TABLE table_24768 ("No. in series" FLOAT, "No. in season" FLOAT, "Title" TEXT, "Directed by" TEXT, "Written by" TEXT, "Original airdate" TEXT, "U.S. viewers  ( millions ) " TEXT)

What number episode in the series was watched by 15.89 million U.S. viewers?
SELECT "No. in series" FROM table_24768 WHERE "U.S. viewers (millions)" = '15.89'
SELECT "No. in series" FROM "table_24768" WHERE "U.S. viewers (millions)" = '15.89'