CREATE TABLE table_1379 ("No. in series" FLOAT, "Title" TEXT, "Directed by" TEXT, "Written by" TEXT, "U.S. viewers  ( million ) " TEXT, "Original air date" TEXT, "Production code" FLOAT)

How many episodes had an original air date with 5.1 million viewers
SELECT COUNT("Original air date") FROM table_1379 WHERE "U.S. viewers (million)" = '5.1'
SELECT COUNT("Original air date") FROM "table_1379" WHERE "U.S. viewers (million)" = '5.1'