A road runs midway between two parallel rows of buildings. A motorist moving with a speed of 36 km/hour sounds the horn. He hears the echo one second after he has sounded the horn. The distance between the 2 rows of buildings is (Velocity of sound is 330 m/s)

Speed of motorist = 36 km/hr = 10 m/s. In 1 second, the motorist moves 10 meters. The sound travels to the wall and back in 1 second. Distance sound travels = 330 * 1 = 330 m. Let d be the distance from the road to one wall. The sound travels (d - 10) + (d + 10) = 2d. So 2d = 330, d = 165. Total distance between walls = 2d = 330 m.