Downstream speed of a swimmer: The speed of a swimmer in still water is 9 km/h. The river flows at 6 km/h. What is the swimmer's downstream speed?

Introduction / Context:
Downstream motion means the swimmer moves with the current. Effective speed increases by the stream speed. This is a direct application of the relative speed model for linear flows.

Given Data / Assumptions:

• Still water speed u = 9 km/h
• Stream speed v = 6 km/h
• Uniform straight current, speeds add linearly downstream

Concept / Approach:
Downstream speed = u + v because both swimmer and current move in the same direction.

Step-by-Step Solution:

Verification / Alternative check:
Upstream would be u - v = 3 km/h, smaller than still water speed. That complements the downstream increase to 15 km/h, confirming logic.

Why Other Options Are Wrong:

• 12 km/h adds partially and is not u + v.
• 3 km/h equals u - v, not downstream.
• 9 km/h ignores current.
• 6 km/h is only the current speed.

Common Pitfalls:
Confusing downstream with upstream or averaging speeds rather than adding for downstream. Always add for downstream and subtract for upstream.

Final Answer:
15 km/h