For long range communication an elephant uses a low frequency rumble that is undetectable to human ears. One elephant can pick up another elephant's call from a distance of 10 km. The frequency of this rumble is about 13 Hz. How fast does the rumble travel?

The speed of sound doesn't depend on the distance of propagation. It depends on the properties of the medium, particulary, on the temperature. So, it is ~ 343 m/s.