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#1 2014-04-02 01:38:05

Agnishom
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From: The Complex Plane
Registered: 2011-01-29
Posts: 18,891
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The Train in the fog

Darjeeling Mail travelling in fog passed a man who was on a bike travelling at 45 km/hr in the same direction. He could see the train for 4 minutes and it is visible to him upto a distance of 100m.
What was the speed of the Darjeeling Mail?

BTW, the train and Darjeeling Mail are the same thing


'And fun? If maths is fun, then getting a tooth extraction is fun. A viral infection is fun. Rabies shots are fun.'
'God exists because Mathematics is consistent, and the devil exists because we cannot prove it'
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#2 2014-04-02 01:42:30

ShivamS
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Registered: 2011-02-07
Posts: 3,629

Re: The Train in the fog

Let x represent the speed of the train.
As I am sure you know, speed = distance / time
x-45 = (100*1/1000 km)/(4*1/60 hours) = 0.1/(1/15)
x = 15*0.1 + 45 = 46.5 km/h

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#3 2014-04-02 01:53:13

Agnishom
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From: The Complex Plane
Registered: 2011-01-29
Posts: 18,891
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Re: The Train in the fog

I do not think I understand the language of the problem fully...


'And fun? If maths is fun, then getting a tooth extraction is fun. A viral infection is fun. Rabies shots are fun.'
'God exists because Mathematics is consistent, and the devil exists because we cannot prove it'
'You have made another human being happy. There is no greater accomplishment.' -bobbym

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#4 2014-04-02 03:26:17

bob bundy
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Registered: 2010-06-20
Posts: 6,532

Re: The Train in the fog

hi Agnishom,

How long is the Darjeeling Mail train?  Without this, I cannot determine an answer.  sad

Bob


You cannot teach a man anything;  you can only help him find it within himself..........Galileo Galilei

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#5 2014-04-02 07:36:22

ShivamS
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Registered: 2011-02-07
Posts: 3,629

Re: The Train in the fog

Why?

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#6 2014-04-02 08:20:29

bob bundy
Moderator
Registered: 2010-06-20
Posts: 6,532

Re: The Train in the fog

hi ShivramS

I like your answer (perfectly reasonable method  smile ) but the information given is insufficient to avoid other interpretations.  It's a train (ie. locomotive plus coaches) and our biker should be using his rear view mirror. 

In my diagram the train is going from left to right.  The biker can first see it at position x and continues to see it until position y.  So, in 4 minutes the train has travelled 200m plus the length of the train.

If you're not meant to take account of L, then why not make it a Harley ?

Even without the rear view the distance is 100 + L .

Bob

View Image: train2.gif

You cannot teach a man anything;  you can only help him find it within himself..........Galileo Galilei

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#7 2014-04-02 08:23:05

ShivamS
Member
Registered: 2011-02-07
Posts: 3,629

Re: The Train in the fog

Many of these questions are not well developed. I remember one Agnishom posted a few weeks ago which had an issue as well.

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