Fred strikes a steel train rail with a hammer at a frequency of 0.52 Hz, and Wilma puts her ear to the rail 2.1 km away. How long after the first

Fred strikes a steel train rail with a hammer at a frequency of 0.52 Hz, and Wilma puts her ear to the rail 2.1 km away. How long after the first strike does Wilma hear the sound? What is the time interval between the successive sound pulses she hears?