Final answer:
b. 20 feet.
The question asks about legal trailer length limits for specific truck combinations.
Step-by-step explanation:
The question pertains to regulations on the maximum allowable length for trailers in certain combinations when operated on highways. While the information provided primarily discusses fuel economy and cargo capacity for long-haul trucks, which is commonly around 20 tons, it doesn't directly provide a specific answer to the question of trailer length limits.
However, in the context of legal regulations for highway vehicle dimensions, which is a matter of law, the maximum length for a trailer or semi-trailer in a truck-semi-trailer-trailer combination is typically specified by law, rather than by the physical characteristics of the trucks themselves.
Therefore, without explicit information on the legal length limitation in the given text, one cannot accurately answer the question. It's important to refer to the relevant state or national transportation regulations for the specific limitations on trailer lengths.