Question: Which episode had bbc ranking and canle ranking of n/a?
Schema: create table table_24399615_10 (episode_no varchar, bbc_three_weekly_ranking varchar, cable_rank varchar, PRIMARY KEY (episode_no))
SQL: select count(episode_no) from table_24399615_10 where bbc_three_weekly_ranking = "n/a" and cable_rank = "n/a"