In one hour, a boat goes 11 km along the stream and 7 km against the stream. the sped of the boat in still water (in km/hr) is : Answers.
A) 3 B) 5 C) 8 D) 9 E) 10

Rationale: "solution speed in still water = 1 / 2 ( 11 + 7 ) km / hr = 9 kmph . answer d"
Answer: D