If anshul rows 15 km upstream and 21 km downstream taking 3 hours each time, then the speed of the stream is :

A. 1 km/hr
B. 2 km/hr
C. 1.5 kin/hr
D. 12 km/hr
Answer: A

Explanation:  

Rate upstream = (15/3) kmph
Rate downstream (21/3) kmph = 7 kmph.
Speed of stream (1/2)(7 - 5)kmph = 1 kmph.