Final answer:
The subject of this question is Physics, as it involves the concepts of relative velocity and the effect of current on a boat's speed.
Step-by-step explanation:
To determine the subject of this question, we look at the given information about a boat traveling both upstream and downstream, and the time it takes for each trip. This question involves the concepts of relative velocity and the effect of current on the boat's speed. Therefore, the subject of this question is Physics.