Calculate the speed of a boat in still water (in km/hr) if in one hour, the boat goes 15 km/hr downstream and 7 km/hr upstream.  Choose the correct choice:
A) 12 kmph
B) 13 kmph
C) 14 kmph
D) 15 kmph
E) 11 kmph

Rationale: "speed in still water = ( 15 + 7 ) 1 / 2 kmph = 11 kmph . answer : e"
Answer: E