A boat goes 40 km upstream in 8 h and 36 km downstream in 6 h. Find the boat’s speed in still water.

Introduction / Context:
Average of upstream and downstream speeds yields still-water speed under constant stream conditions.

Given Data / Assumptions:

• Upstream speed u = 40/8 = 5 km/h.
• Downstream speed d = 36/6 = 6 km/h.
• Still-water speed b = (d + u)/2.

Concept / Approach:
Set u = b − s and d = b + s; adding gives 2b = u + d.

Step-by-Step Solution:

Verification / Alternative check:
Then s = (d − u)/2 = 0.5 km/h; reconstruct u and d to confirm.

Why Other Options Are Wrong:
6 or 6.5 contradict the arithmetic mean.

Common Pitfalls:
Using the harmonic mean instead of the direct average (not applicable here).

Final Answer:
5.5 km/hr