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A boat covers a distance from A to B downstream and B to A upstream in 4 hours. If the speed of the stream is 2 km/hr and the speed of the boat in still water is 4 km/hr, then the distance between A and B is :


1. 8 km
2. 9 km
3. 4 km
4. 6 km

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Correct Answer - Option 4 : 6 km

Given  

The boat covers a distance from A to B downstream and from B to A upstream in 4 hours  

The speed of the stream = 2 km/hr

The speed of the boat in still water = 4 km/hr,

Concept Used:

Upstream speed = Speed of boat - Speed of the stream

Downstream speed = Speed of boat + Speed of the stream

Calculation :

Speed of boat downstream = (4 + 2) km/hr = 6 km/hr

speed of boat upstream = (4 - 2) km/hr = 2 km/hr

Let, the distance between point  and point  be  km 

Accordingly,   

x/6 + x/2 = 4

⇒ (x + 3x)/6 = 4  (cross multiple)

⇒ 4x = 24 

⇒ x = 24/ 4 

⇒ x = 6

∴  The distance between A and B is 6 km

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