Find stream speed from given up/down speeds: A man rows upstream at 15 km/h and downstream at 21 km/h. What is the speed of the current?

Introduction / Context:
Upstream and downstream speeds are symmetric around the still-water speed. The current speed is simply half the difference between downstream and upstream speeds.

Given Data / Assumptions:

• Upstream = 15 km/h.
• Downstream = 21 km/h.
• Let v be the current speed.

Concept / Approach:
v = (down − up)/2; u = (down + up)/2 (for reference).

Step-by-Step Solution:
v = (21 − 15) / 2 = 3 km/h.u = (21 + 15) / 2 = 18 km/h (still-water speed).

Verification / Alternative check:
Check: u + v = 18 + 3 = 21; u − v = 18 − 3 = 15, matching the given.

Why Other Options Are Wrong:
6, 5, 4, 8 are not the half-difference here.

Common Pitfalls:
Using full difference instead of half; adding instead of subtracting.

Final Answer:
3 km/h