Determine the average gross for Disney's PG-13-rated action movies.
DIVIDE(SUM(total_gross where genre = 'Action' and MPAA_rating = 'PG-13'), COUNT(movie_title where genre = 'Action' and MPAA_rating = 'PG-13'));
SELECT SUM(CAST(REPLACE(trim(total_gross, '$'), ',', '') AS REAL)) / COUNT(movie_title) FROM movies_total_gross WHERE MPAA_rating = 'PG-13'