CREATE TABLE table_26438 ("No. by series" FLOAT, "No. by season" FLOAT, "Title" TEXT, "Directed by" TEXT, "Written by" TEXT, "Original air date" TEXT, "Production number" FLOAT, "U.S. viewers  ( in millions ) " TEXT)

Name the viewers for 520
SELECT "U.S. viewers (in millions)" FROM table_26438 WHERE "Production number" = '520'
SELECT "U.S. viewers (in millions)" FROM "table_26438" WHERE "Production number" = '520'