Final answer:
The federal requirement for the annual safety inspection of trailers in the U.S. is for those with a GVWR of 10,000 pounds or more, though local laws may differ. Overloading a water taxi, as in the example provided, would indeed pose a safety risk.
Step-by-step explanation:
The question regarding the weight threshold for safety inspections falls under the domain of law and regulations about transportation safety, rather than academic school subjects. Without specific regulatory details, it's assumed that the question is about a legal safety requirement for trailers. If this query is referring to the United States, for instance, the federal government, through the Department of Transportation, requires commercial trailers with a gross vehicle weight rating (GVWR) of 10,000 pounds or more to be inspected annually. However, trailer inspection requirements can vary by state or country, hence it is important to check local laws and regulations for accuracy.
To address the given scenario about safety concerns for water taxis: If 20 men have a sum weight greater than 3,500 pounds, and this weight exceeds the safety limits for water taxis, then there is indeed a safety concern. Overloading a water taxi beyond its safety limits could result in a higher risk of accidents or even capsizing, which can lead to injuries or fatalities.