Math Is Fun Forum

Agnishom

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From: Riemann Sphere

Registered: 2011-01-29

Posts: 25,002

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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

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'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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ShivamS

Member

Registered: 2011-02-07

Posts: 3,648

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

Agnishom

Real Member

From: Riemann Sphere

Registered: 2011-01-29

Posts: 25,002

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Re: The Train in the fog

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

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'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'
I'm not crazy, my mother had me tested.

Bob

Administrator

Registered: 2010-06-20

Posts: 10,643

Re: The Train in the fog

hi Agnishom,

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

Bob

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Children are not defined by school ...........The Fonz
You cannot teach a man anything;  you can only help him find it within himself..........Galileo Galilei
Sometimes I deliberately make mistakes, just to test you!  …………….Bob

ShivamS

Member

Registered: 2011-02-07

Posts: 3,648

Re: The Train in the fog

Why?

Bob

Administrator

Registered: 2010-06-20

Posts: 10,643

Re: The Train in the fog

hi ShivramS

I like your answer (perfectly reasonable method  ) 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

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Children are not defined by school ...........The Fonz
You cannot teach a man anything;  you can only help him find it within himself..........Galileo Galilei
Sometimes I deliberately make mistakes, just to test you!  …………….Bob

ShivamS

Member

Registered: 2011-02-07

Posts: 3,648

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.