What was the question for this implicit rationale, and corresponding answer?
Speed in still water = (10 + 4)1/2 kmph = 7 kmph.
The answer: (A)
The question: Calculate the speed of a boat in still water (in km/hr) if in one hour, the boat goes 10 km/hr downstream and 4 km/hr upstream.
Options:
(A) 7 kmph
(B) 8 kmph
(C) 9 kmph
(D) 7 kmph
(E) 6 kmph