Calculate the speed of a boat in still water (in km/hr) if in one hour, the boat goes at 13 km/hr downstream and 4 km/hr upstream. Choose the correct option.
A) 4.5 kmph  B) 9.5 kmph  C) 7.5 kmph  D) 8.5 kmph  E) 5.5 kmph

Rationale: "speed in still water = ( 13 + 4 ) 1 / 2 kmph = 8.5 kmph . answer : d"
Answer: D