A man rows to a place at a distance of 72 km and comes back in 36 hours. He finds that he can row 12 km with the stream in the same time as 4 km against the stream. The speed (in km/h, rounded off to 2 decimal places) of the stream is:

Correct option is C

$$\frac{72}{3a} + \frac{72}{a}= 36$$​Let the speed of boat and water be x km/hr and y km/hr.

​$$\frac {12}{x+y} = \frac{4}{x-y}$$

12 x - 12 y = 4x+ 4y 

8x =  16y 

x/y = 2/1 

So, the speed of boat and water be 2a km/hr and a km/hr.

Given,

​$$\frac{72}{x+y} + \frac{72}{x-y} = 36$$ 

​$$\frac{72}{3a} + \frac{72}{a} = 36$$

On Solving a= 2.67 km/hr

Hence, speed of stream = 2.67 km/hr