CREATE TABLE table_1839 ("No. in series" FLOAT, "No. in season" FLOAT, "Title" TEXT, "Directed by" TEXT, "Written by" TEXT, "Original air date" TEXT, "Production code" TEXT, "U.S. viewers  ( million ) " TEXT)

How many episodes by different writers were seen by 3.24 million people in the US?
SELECT COUNT("Written by") FROM table_1839 WHERE "U.S. viewers (million)" = '3.24'
SELECT COUNT("Written by") FROM "table_1839" WHERE "U.S. viewers (million)" = '3.24'