A boat moves 25 km upstream and 39 km downstream in 8 hours. It travels 35 km upstream and 52 km downstream in 11 hours. What is the speed of the stream if it travels at a uniform speed?

Answer: Option A

Let speed of boat is x and stream is y km/hr
According to the question
$$\begin{array}{rl}& \frac{25}{x-y}+\frac{39}{x+y}=8\\ & \frac{35}{x-y}+\frac{52}{x+y}=11\end{array}$$
By hit and trail method x = 9, y = 4 it satisfy the both equations
Speed of stream = 4 km/hr