What is the average page number of the menus that have the dish "Clear green turtle"?
average page number = AVG(page_number); Clear green turtle is a name of dish;
SELECT AVG(T1.page_number) FROM MenuPage AS T1 INNER JOIN MenuItem AS T2 ON T1.id = T2.menu_page_id INNER JOIN Dish AS T3 ON T2.dish_id = T3.id WHERE T3.name = 'Clear green turtle'