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 ( num_employees INTEGER, ranking INTEGER )

### 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