CREATE TABLE table_20910 ("No. in series" TEXT, "No. in season" TEXT, "Title" TEXT, "Directed by" TEXT, "Written by" TEXT, "Original airdate" TEXT, "Production code" TEXT)

What is the series episode number of the episode with production code 303?
SELECT "No. in series" FROM table_20910 WHERE "Production code" = '303'
SELECT "No. in series" FROM "table_20910" WHERE "Production code" = '303'