A boat has speed 4 km/h in still water and the stream speed is 2 km/h. It takes 9 hours to go a certain distance upstream. How long will it take to come back the same distance downstream?

Introduction / Context:
Given still-water and current speeds, we compute upstream and downstream speeds and transfer time information from one direction to the other for the same distance.

Given Data / Assumptions:

• b = 4 km/h, c = 2 km/h.
• Upstream speed = b - c = 2 km/h; downstream speed = b + c = 6 km/h.
• Upstream time = 9 h over distance D.

Concept / Approach:
Distance D = speed * time. Then use D to find the downstream time with faster effective speed.

Step-by-Step Solution:
D = upstream speed * time = 2 * 9 = 18 km Downstream time = D / (b + c) = 18 / 6 = 3 h

Verification / Alternative check:
Round trip check: 9 h up + 3 h down = 12 h total; numbers are consistent and units align.

Why Other Options Are Wrong:
2 or 2.5 h are too short for 18 km at a maximum of 6 km/h; 3.5 h is too long.

Common Pitfalls:
Using 4 km/h or 2 km/h directly for downstream; forgetting upstream is slower and downstream is faster; unit mismatches.

Final Answer:
3 hrs