A boat can go 72 km downstream in 4 hours and 36 km upstream in 3 hours. Calculate speed of boat in still water.

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15 km/hr

12 km/hr

18 km/hr

24 km/hr

answer is A.

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Detailed Solution

Concept-
It is called upstream if the boat is moving in the opposite direction of the stream. The net speed of the boat is referred to as the upstream speed in this case. It is called downstream if the boat is moving in the direction of the stream. The net speed of the boat in this case is referred to as downstream speed.
Speed of a boat in still water

( upstream speed+ downstream speed)
Here, the boat can go

downstream in 4 hours.
Then, downstream speed

It is also given that the boat can go

downstream in 3 hours.
Then, upstream speed

Speed in still water

Therefore, the speed of boat in still water is

.
Hence, the correct answer is 1) 15 km/hr.

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