In one hour, a boat goes 15 km along the stream and 5 km against the stream. the sped of the boat in still water (in km/hr) is :
Choose the correct choice from the following choices:
A) 3  B) 5  C) 8  D) 9  E) 10

Rationale: "solution speed in still water = 1 / 2 ( 15 + 5 ) km / hr = 10 kmph . answer e"
Answer: E