In one hour,a boat goes 13 km along the stream and 5 km against the stream.The speed of the boat in still water (in km/hr) is :  Pick one:
A) 2 B) 4 C) 9 D) 12 E) 15

Rationale: "sol . speed in still water = 1 / 2 ( 13 + 5 ) kmph = 9 kmph . answer c"
Answer: C