Final answer:
Without specific details about the double track's length and the tractor-trailer's speed, it is not possible to determine the exact time it takes to clear the track. Generally, it can take more than 15 to 30 seconds based on typical tractor-trailer speeds and track lengths.
Step-by-step explanation:
The time it takes for a normal tractor-trailer to clear a double track can vary based on the speed at which the tractor-trailer is moving and the length of the tracks.
Since the specific length of the double track is not given, it is not possible to accurately determine how many seconds it would take for the tractor-trailer to clear the tracks without additional information. Typical tractor-trailers, while properly moving at posted speed limits, can take anywhere around more than 15 seconds to 30 seconds or even longer, depending on factors such as the vehicle's speed and length of the trailer.
The information provided also indicates the need to understand concepts such as velocity, distance traveled before braking, and braking distance, which are crucial in calculations involving moving vehicles.
For example, knowing the velocity a car travels through a valley or the distance it covers before hitting the brakes contributes to understanding the vehicle's overall stopping distance in relation to reaction time and braking efficiency.