Ashutosh can row at 24 km/h in still water. He takes twice as long to row a fixed distance upstream as he takes to row the same distance downstream. What is the speed of the stream (in km/h)?

Introduction / Context:
In boats and streams, the effective speed differs depending on whether motion is with the current (downstream) or against it (upstream). When the time relation between upstream and downstream for the same distance is given, we can form an equation directly in terms of the still water speed and the current speed.

Given Data / Assumptions:

• Still water speed v = 24 km/h.
• For the same one-way distance D, time upstream is twice time downstream.
• Uniform current and no delays at turns.

Concept / Approach:
Downstream speed = v + c; upstream speed = v − c. For the same D, time is inversely proportional to speed. Condition: D/(v − c) = 2 * D/(v + c). Cancel D to obtain an equation in v and c.

Step-by-Step Solution:

Verification / Alternative check:
With c = 8: downstream speed = 24 + 8 = 32; upstream speed = 24 − 8 = 16. For distance D, times are D/32 and D/16, whose ratio is 1:2 as required.

Why Other Options Are Wrong:
4 and 15 or 18 km/h do not satisfy 3c = v when v = 24 and hence do not produce the required 2:1 time ratio.

Common Pitfalls:
Equating times to the ratio of speeds directly or averaging speeds arithmetically. The time relationship must be set up via reciprocals of speeds.

Final Answer:
8 km/h