In the film with an inventory ID between 20 to 60, how many of the films have a G rating?
G rating refers to rating = 'G'
SELECT COUNT(T1.film_id) FROM film AS T1 INNER JOIN inventory AS T2 ON T1.film_id = T2.film_id WHERE T2.inventory_id BETWEEN 20 AND 60 AND T1.rating = 'G'