Final answer:
The situation where distance traveled is proportional to time is when a person runs at a constant speed on a treadmill, such as at 3 mph for 6 minutes, because the relationship between distance and time is linear.
Step-by-step explanation:
In the context of the question, the situation in which the distance traveled is proportional to time is when a person is running on a treadmill at a constant speed, specifically at 3 mph for 6 minutes. This is because when the speed is constant, the distance traveled over time is a linear relationship, which is what defines proportionality.
For instance, as mentioned, if you travel 10 km in a third of an hour (20 minutes), logically you would cover three times that distance in a full hour, assuming the speed remains unchanged.
Therefore, the person running on a treadmill represents a scenario where speed is constant and, thus, the distance is proportional to the time.