A boatman goes 2 km against the current of the stream in 1 hour and goes 1 km along the current in 10 minutes.
How long will it take to go 5 km in stationary water?

Let us assume that the speed of boat = B and speed of stream = S

Now, it is given that A boatman goes 2 km against the stream's current in 1 hour.

Net speed of boat against the stream (upstream) = B-S

(B-S)*1 = 2 

B-S = 2   -----equation (1)

Now, it is also given that the boat goes 1 km along the current in 10 minutes.

The net speed of the boat in the direction of the stream (downstream) = B+S

(B+S)*1/6 = 1 (As 10 minutes = 1/6 hour)

B+S = 6   ------equation (2)

Adding both, we get 2B = 8 or B = 4 km/hr

Thus, S = 2 km/hr

We need to find a boat's time to go 5 km in stationary water.

Distance/Speed = Time

5 km/4 km/hr = 1 hour 15 minutes