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 department ( department_id number, name text, creation text, ranking number, budget_in_billions number, num_employees number )
TABLE: CREATE TABLE management ( department_id number, head_id number, temporary_acting text )
TABLE: CREATE TABLE head ( head_id number, name text, born_state text, age number )

### Question ###
What is the average number of employees of the departments whose rank is between 10 and 15?

### Accurate SQL ###

SELECT AVG(num_employees) FROM department WHERE ranking BETWEEN 10 AND 15