Below is the list of tables from the database along with their columns and datatype. Need to generate an accurate SQL query for the question.
Only use the columns present in the respective table. Only use the columns which are required to generate the SQL query. Only use table joins if required and not otherwise.

### Tables ###
TABLE: CREATE TABLE manufacturer ( Num_of_Factories INTEGER, num_of_shops INTEGER )

### Question ###
Find the average number of factories for the manufacturers that have more than 20 shops.

### Accurate SQL ###

SELECT AVG(Num_of_Factories) FROM manufacturer WHERE num_of_shops > 20