4.9 Two cars approach each other. The first car travels at 20 m/s and the other at 15 m/s. The first car suddenly blows its horn emitting sound waves with frequency of 700 Hz. (a) What frequency is perceived by an observer in the second car? (b) What frequency is perceived by an observer in the second car as the cars pass each other? (The temperature of the surrounding is 25 C.)