Question

Speed of a boat in still water is 15km/h. It goes 30 km upstream and returns back at the same point in 4 hours 30
minutes. Find the speed of the stream.
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Answer 1

Speed of boat = 15 km/hr

Distance = 30 km.

Let, Speed of the stream = x km/hr

Total time = 4 hours 30 minutes = 4 (1/2) = 9/2.

It goes 30 km upstream.

Speed of upstream = (15 - x) km/hr

Time taken for upstream = (30/15 - x)

It goes 30 km downstream.

Speed of downstream = (15 + x) km/hr

Time taken for downstream = (30/15 + x)

Total time = Time taken for downstream + upstream

=> 9/2 = (30/15 + x) + (30/15 - x)

=> 9/2 = 30(30/225 - x²)

=> 225 - x² = 200

=> x² = 25

⇒ x = 5, -5[Speed cannot be negative]

=> x = 5 km/hr

Hence, Speed of the stream = 5 km/hr

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