Discussion :: Boats and Streams

What is the speed of the boat in still water?
I. The speed downstream is 12 kmph.
II.The speed upstream is 4 kmph.
III.In a to and fro journey between two points, the average speed of the boat was 6 kmph.
A.
I and II only 
B.
All I, II and III 
C.
III, and either I or II 
D.
Any two of the three 
E.
None of these 
Answer : Option D
Explanation :
From I and II, speed of boat in still water =  1  (12 + 4) km/hr = 8 km/hr. 
2 
From II and III, we get:
Using average speed=\(\frac { 2XY } { X+Y } \),we get:\( \frac { 2*4*Y} {4+Y} =6\)
y = 12.
Required speed =\(\frac { 1 } { 2 }\)(12 + 4) km/hr = 8 km/hr.
Similarly, I and III also give the answer.
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