A man can row a boat at speed 8 km/h in still water. It takes thrice as long to row upstream as it does to row downstream. Find the speed of the stream (in km/h).

Correct option is B

Given:
Speed of boat in still water = 8km/hr
Time taken upstream is thrice the time taken downstream.
Formula Used:
Let the speed of the stream be x km/hr
Upstream speed = (8-x) km/ hr
Downstream speed = (8+x) km/ hr
Time = Distance/Speed
Solution:
Let the distance be d.
Time taken upstream = $$\frac{d}{8-x}$$​​
Time taken downstream = $$\frac{d}{8+x}$$​​
Given time taken upstream is thrice time taken downstream:
​$$\frac{d}{8-x} = 3 \cdot \frac{d}{x+8}\\\frac{1}{8-x} = 3 \cdot \frac{1}{x+8}\\x + 8 = 24 - 3x\\\\16 = 4x\\x = 4 \, \text{km/hr}\\$$

Alternate Method:

Let the time taken in upstream and downstream be 3x and x respectively

Speed in the upstream and downstream will be x and 3x

Man's speed in still water = (x + 3x)/2

=> 16 = 4x

=> x = 4

The stream velocity = $$\frac{\text{Speed in downstream} - \text{Speed in upstream}}{2}$$​​

The stream velocity = $$\frac{3x - x}{2} = 4 km/h$$​​