In one hour a boat goes 11 km along the stream and 5 km against the stream. The speed of the boat in still water in km/hr is:

Explanation

Downstream speed = 11 km/hr and upstream speed = 5 km/hr. The speed of the boat in still water is their average:

(11+5)/2=8 km/hr.
For comparison, the stream speed is (11-5)/2=3 km/hr.