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Re: A motorboat whose speed is 20 Kmph in still water goes 30 km downstrea [#permalink]
tapas3016 wrote:
Time = DistanceSpeed

5 = TotalDistancetravelledSpeedinstillwater+Speedinstream + TotalDistancetravelledSpeedinstillwaterSpeedinstream

Let the speed in stream be X kmph

5 = 3020+x +3020x

On Solving, We get

400 -x2= 240
x2 = 160

Taking Square root, it will be approx 4*3.16 which is greater than 10.

Hence A

Sir, I did not understand this formula. Thanks in Advance :heart
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A motorboat whose speed is 20 Kmph in still water goes 30 km downstrea [#permalink]
Let's take a speed of boat in a still water = x & speed of current = y

Now, when the boat travels downstream, the relative speed = |x+y|
& when the boat travels upstream, the relative speed = |xy|


kumarneupane4344 wrote:
tapas3016 wrote:
Time = DistanceSpeed

5 = TotalDistancetravelledSpeedinstillwater+Speedinstream + TotalDistancetravelledSpeedinstillwaterSpeedinstream

Let the speed in stream be X kmph

5 = 3020+x +3020x

On Solving, We get

400 -x2= 240
x2 = 160

Taking Square root, it will be approx 4*3.16 which is greater than 10.

Hence A

Sir, I did not understand this formula. Thanks in Advance :heart
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Re: A motorboat whose speed is 20 Kmph in still water goes 30 km downstrea [#permalink]
rx10 wrote:
Let's take a speed of boat in a still water = x & speed of current = y

Now, when the boat travels downstream, the relative speed = |x+y|
& when the boat travels upstream, the relative speed = |xy|


kumarneupane4344 wrote:
tapas3016 wrote:
Time = DistanceSpeed

5 = TotalDistancetravelledSpeedinstillwater+Speedinstream + TotalDistancetravelledSpeedinstillwaterSpeedinstream

Let the speed in stream be X kmph

Thank you :blushing: :blushing:

5 = 3020+x +3020x

On Solving, We get

400 -x2= 240
x2 = 160

Taking Square root, it will be approx 4*3.16 which is greater than 10.

Hence A

Sir, I did not understand this formula. Thanks in Advance :heart
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Re: A motorboat whose speed is 20 Kmph in still water goes 30 km downstrea [#permalink]
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kumarneupane4344

Let me reply on his behalf!

Let the speed of boat be b and that of the stream be x

Total Time taken for the trip = Time taken to travel downstream + Time taken to travel upstream

Time taken to travel downstream = db+x=3020+x

Time taken to travel upstream = dbx=3020x

We have been given total time as five hours. So,

5=3020+x+3020x
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Re: A motorboat whose speed is 20 Kmph in still water goes 30 km downstrea [#permalink]
KarunMendiratta wrote:
kumarneupane4344

Let me reply on his behalf!

Let the speed of boat be b and that of the stream be x

Total Time taken for the trip = Time taken to travel downstream + Time taken to travel upstream

Time taken to travel downstream = db+x=3020+x

Time taken to travel upstream = dbx=3020x

We have been given total time as five hours. So,

5=3020+x+3020x



Thank you sir. Kudos!
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Re: A motorboat whose speed is 20 Kmph in still water goes 30 km downstrea [#permalink]
BrushMyQuant wrote:
Theory

Downstream: If a boat with speed v in still water is going down on a stream of water which is flowing at speed u then the speed of the boat downstream will become v + u

Upstream: If a boat with speed v in still water is going up on a stream of water which is flowing at speed u then the speed of the boat Upstream will become v - u

Let Speed of the stream is u kmph

Downstream
Distance = 30 km
Speed = 20 + u kmph
Time = DistanceSpeed = 30km20+ukmph
= 3020+u hours

Upstream
Distance = 30 km
Speed = 20 - u kmph
Time = DistanceSpeed = 30km20ukmph
= 3020u hours

Total Time Downstream + Upstream = 3020+u hours + 3020u hours = 5 hours [given]

=> 30*(20-u) + 30*(20+u) = 5*(20-u)*(20+u) [ Divide both sides by 5 we get]
=> 6*(20-u) + 6*(20+u) = 202 - u2 [ Using (a-b)*(a+b) = a2 - b2 ]
=> 120 - 6u + 120 + 6u = 400 - u2
=> 240 = 400 - u2
=> u2 = 400 - 240 = 160
=> u = 12.64

=> Quantity A (12.64) > Quantity B (10)

So, answer will be A
Hope it helps!



Well explanation, Thank you ankit sir!
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Re: A motorboat whose speed is 20 Kmph in still water goes 30 km downstrea [#permalink]
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Re: A motorboat whose speed is 20 Kmph in still water goes 30 km downstrea [#permalink]
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