A boat takes 4 hours for travelling downstream from point A to point B aild coming back to point A upstream. If the velocity of the stream is 2 kmph and the speed of the boat in still water is 4 kmph, what is the distance between A and B?

A. 4 kms
B. 8 km
C. 6 km
D. 9 km
Answer: C

Explanation:  

Let the distance between A and B be x km.
Speed downstream = 6 kmph, speed upstream = 2 kmph.
(x/6) + (x/2) = 4 4x = 24 hence x = 6
Distance AB = 6 km.