In one hour,a boat goes 21 km along the stream and 9 km against the stream.The speed of the boat in still water (in km/hr) is :	Choose the correct option:
A) 3	B) 5	C) 10	D) 9	E) 15

Rationale: "solution speed in still water = 1 / 2 ( 21 + 9 ) kmph . = 15 kmph . answer e"
Answer: E