CREATE TABLE table_15938543_1 (production_code VARCHAR, written_by VARCHAR)

How many episodes were written by Jack Burditt & Robert Carlock
SELECT COUNT(production_code) FROM table_15938543_1 WHERE written_by = "Jack Burditt & Robert Carlock"
SELECT COUNT("production_code") FROM "table_15938543_1" WHERE "Jack Burditt & Robert Carlock" = "written_by"