The City of Charleston Fire Department consists over 300 full-time firefighters. These firefighters operate out of 20 companies located throughout the city: 16 engine companies, two tower companies, and one ladder company. Training, Fire Marshall, Operations, and Administration are the divisions of the department. The department operates on a 24/48 schedule and had a Class 1 ISO rating until late 2008, when ISO officially lowered it to Class 3.  Russell  Thomas served as Fire Chief until June 2008, and was succeeded by Chief Thomas Carr in November 2008.
Question: How many more engine companies are there than ladder companies?
Answer: 15