SELECT avg(Num_of_Factories) FROM manufacturer WHERE num_of_shops  >  20
Find the average number of factories for the manufacturers that have more than 20 shops.
[Schema (values) (types)]: | manufacturer |  manufacturer : manufacturer_id (text) , open_year (number) , name (number) , num_of_factories (text) , num_of_shops (number) | furniture : furniture_id (text) , name (number) , num_of_component (number) , market_rate (text) | furniture_manufacte : manufacturer_id (text) , furniture_id (number) , price_in_dollar (number);