Final answer:
The question appears to be about traffic regulations for a railroad crossing with limited visibility, but without specific regional laws, the safest answer would be to follow the lowest speed option provided.
Step-by-step explanation:
The correct answer to the question about the speed limit when approaching a railroad crossing with no warning devices and limited visibility does not directly pertain to the example physics problems given, such as the speed of sound, the frequency of a train's horn, or calculating relative speed of a man to a train.
Rather, it is a question about driving safety and traffic regulations. Generally, rules of the road dictate that when approaching a railroad crossing with poor visibility, one should slow down to ensure safety.
However, without the specific traffic regulations of the area being mentioned or a clear connection to the physics examples provided, we cannot accurately determine the exact speed limit applicable in this scenario.
For safety, one should choose the lowest speed option given to ensure ample time to react to any oncoming trains.
The correct answer is option 15. When approaching a railroad crossing with no warning devices and unable to see 400 feet down the tracks, the speed limit is typically 15 mph according to safety regulations.
This reduced speed limit allows drivers to have sufficient time to detect an approaching train and stop safely if necessary. By traveling at a slower speed, drivers can also better assess the situation and make informed decisions.