Question

A boat covers a distance downstream in 4 hr and re-
turn at original place upstream is 5 hr. If speed of boat
in still water is 18 km/h. Find the speed of stream.l​

Answer 1

Let :

• Speed of stream = v km/h
• Distance covered on one side = x km

Given :

• Speed of boat in still water = 18 km/h
• Time taken to cover distance'x' upstream = 5hr
• Time taken to cover distance'x' downstream = 4hr

To find :

Speed of stream

SOLUTION :

1) Downstream -

Distance = x km

Time = 4hr

Net speed of boat downstream = speed of boat in still water + speed of stream

Net speed of boat downstream = 18km/h + v km/h

Now using , Distance = speed × time

We get,

➝ x = (18+v)× 4 equation 1

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2) Upstream

Distance = x km

Time = 5 hr

Net speed of boat upstream = speed of boat in still water - speed of stream

Net speed of boat upstream = 18 km/h - v km/h

Now using , Distance = speed × time

We get,

➝ x = (18-v) × 5 equation 2

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On putting value of x from equation 1 into equation 2 we get;

➝ 4×(18+v) = 5×(18-v)

➝ 72 + 4v = 90 - 5v

➝ 5v + 4v = 90 - 72

➝ 9v = 18

➝ v = 18/9

➝ v = 2

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ANSWER :

Speed of stream = 2 km/h