Math Is Fun Forum

Aishwarya Krishnaswamy

Member

Registered: 2011-05-14

Posts: 11

An interesting problem

Hi all,

I found this problem in a personality development test.

A train which is 100m long starts running at the speed of 70km/hr. A man also starts running at 10 kms/hr. In how much time does the train cross the man.

I got an approximate answer of 6 seconds when i tried to compute it by taking when the train covers 100 m then the man would have run for 15 m. Then when the train runs for 15 m, the man would have run 1.67 m and so on till I neared zero and got the answer as 6 seconds.

Could you tell me if there is a better way to do it?

Regards
Aishwarya

anonimnystefy

Real Member

From: Harlan's World

Registered: 2011-05-23

Posts: 16,049

Re: An interesting problem

hi aishwarya,

you can do it like an equation.

which means that the train has to go x seconds to cover the distance that the man will go plus its own 100m=0,1km.
from this you get

hope it helps.

Last edited by anonimnystefy (2011-05-25 22:17:30)

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“Here lies the reader who will never open this book. He is forever dead.
“Taking a new step, uttering a new word, is what people fear most.” ― Fyodor Dostoyevsky, Crime and Punishment
The knowledge of some things as a function of age is a delta function.

gAr

Member

Registered: 2011-01-09

Posts: 3,482

Re: An interesting problem

Hi,

Relative velocity is 60km/hr.
Time to move 100m:

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"Believe nothing, no matter where you read it, or who said it, no matter if I have said it, unless it agrees with your own reason and your own common sense"  - Buddha?

"Data! Data! Data!" he cried impatiently. "I can't make bricks without clay."

anonimnystefy

Real Member

From: Harlan's World

Registered: 2011-05-23

Posts: 16,049

Re: An interesting problem

hi gAr,
Ihave one for you:
we have two trains on the distance of 300 meters.On one fly there is a fly traveling from one train to another with velocity of 30m per second.The trains are going towards each other one with velocity of 10m per second,and the other with velocity of 20 m per second.What's the overall distance that the fly will overcome until the trains crash into each other?

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“Here lies the reader who will never open this book. He is forever dead.
“Taking a new step, uttering a new word, is what people fear most.” ― Fyodor Dostoyevsky, Crime and Punishment
The knowledge of some things as a function of age is a delta function.

bobbym

bumpkin

From: Bumpkinland

Registered: 2009-04-12

Posts: 109,606

Re: An interesting problem

Hi anonimnystefy;

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In mathematics, you don't understand things. You just get used to them.
If it ain't broke, fix it until it is.
Always satisfy the Prime Directive of getting the right answer above all else.

anonimnystefy

Real Member

From: Harlan's World

Registered: 2011-05-23

Posts: 16,049

Re: An interesting problem

---

“Here lies the reader who will never open this book. He is forever dead.
“Taking a new step, uttering a new word, is what people fear most.” ― Fyodor Dostoyevsky, Crime and Punishment
The knowledge of some things as a function of age is a delta function.

gAr

Member

Registered: 2011-01-09

Posts: 3,482

Re: An interesting problem

Hi anonimnystefy,

---

"Believe nothing, no matter where you read it, or who said it, no matter if I have said it, unless it agrees with your own reason and your own common sense"  - Buddha?

"Data! Data! Data!" he cried impatiently. "I can't make bricks without clay."

anonimnystefy

Real Member

From: Harlan's World

Registered: 2011-05-23

Posts: 16,049

Re: An interesting problem

Last edited by anonimnystefy (2014-06-14 08:05:41)

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“Here lies the reader who will never open this book. He is forever dead.
“Taking a new step, uttering a new word, is what people fear most.” ― Fyodor Dostoyevsky, Crime and Punishment
The knowledge of some things as a function of age is a delta function.