In one hour, a boat goes 15 km along the stream and 5 km against the stream. The speed of the boat in still water (in km/hr) is:  Options
A) 2	B) 4	C) 10	D) 12	E) 15

Rationale: "sol . speed in still water = 1 / 2 ( 15 + 5 ) kmph = 10 kmph . answer c"
Answer: C