CREATE TABLE table_29977 ("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)

What was the production code of the episode no. 55 in the series?
SELECT "Production code" FROM table_29977 WHERE "No. in series" = '55'
SELECT "Production code" FROM "table_29977" WHERE "No. in series" = '55'