Jennie drives to her son's house in 5 hours when there is heavy traffic. When there is no traffic the same trip takes only 4 hours. If her son's house is 200 miles away, what is the difference between her average speed when there is heavy traffic and when there is no traffic? Let's think first. Chain of thought:
Jennie can cover the 200 - mile distance in 5 hours when there is heavy traffic, so her average speed is 200 / 5 = 40mph. She can cover the same distance in 4 hours when there is no traffic for an average speed of 200 / 4 = 50mph. The difference in average speed between the two scenarios is (50 - 40)mph = 10mph.
Therefore, the answer is 10.