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 A and point B be x 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