What can be deduced from a time relation Tu - Td = 3?

The equation Tu - Td = 3 indicates that the time taken to travel upstream (Tu) is 3 units longer than the time taken to travel downstream (Td). This directly suggests that the upstream journey requires more time compared to the downstream journey.

To interpret this relation, consider that "upstream" typically means moving against the current, which generally slows down the travel time, while "downstream" indicates movement with the current, thereby facilitating faster travel. Since the equation shows a positive difference where Tu exceeds Td by 3, it confirms that the upstream time is indeed longer than the downstream time.

This conclusion about the relative lengths of travel times provides a clear understanding of the dynamics involved in different river currents and travel behaviors. Thus, the statement that the time upstream is longer is accurately supported by the given time relation.