CREATE TABLE table_16537 ("No. in series" FLOAT, "No. in season" FLOAT, "Title" TEXT, "Directed by" TEXT, "Written by" TEXT, "Original air date" TEXT, "U.S. viewers  ( millions ) " TEXT)

How many millions of viewers watched episode 15?
SELECT "U.S. viewers (millions)" FROM table_16537 WHERE "No. in season" = '15'
SELECT "U.S. viewers (millions)" FROM "table_16537" WHERE "No. in season" = '15'