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Igniter Cords


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28 replies to this topic

#1 rookie1ja

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Posted 30 March 2007 - 05:40 PM

Burning Fuses - Back to the Water and Weighing Puzzles
Your job is to measure 45 minutes, if you have only two cords and matches to light the cords.

1. The two cords are twisted from various materials and so their different segments can burn at different rates.
2. Each cord burns from end to end in exactly one hour.

Describe your way of measuring the 45 minutes.

This old topic is locked since it was answered many times. You can check solution in the Spoiler below.
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Spoiler for old wording

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#2 sunil_h_v

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Posted 26 May 2007 - 04:18 PM

well the above reply says start ignition on both ends of the first cord.
The question specifies that it is being built from different materials , so how do we know
that it is exactly symmetrical materials are used from both points such that it will take exactly
30 minutes to reach the end ????

I suppose that answer will not be satiable.
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#3 rookie1ja

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Posted 26 May 2007 - 05:31 PM

well the above reply says start ignition on both ends of the first cord.
The question specifies that it is being built from different materials , so how do we know
that it is exactly symmetrical materials are used from both points such that it will take exactly
30 minutes to reach the end ????

I suppose that answer will not be satiable.


It seems quite clear to me. Second condition says:
2. Every cord burns from ignition to the end exactly one hour.
So if you ignite the same cord on both ends, it should stop burning after half an hour. And, of course, spot where the fire from both ends meets does not have to be in the middle at all.
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#4 sunil_h_v

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Posted 26 May 2007 - 05:46 PM

yes sir, i got your point, my mistake, i dint understand it, apologies for that.

Regards,

Sunil.
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#5 larryhl

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Posted 05 June 2007 - 07:14 PM

that was the extra credit on one of my calc 3 exams. i loved the puzzle then, still a great puzzle now. incidentally, i got it right on the exam =D
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#6 Domhnall

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Posted 06 June 2007 - 08:01 PM

could you not just fold one cord in half then make note of the middle, then fold that half in the middle and make note of the quarter, then light and wait for that quarter to burn?
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#7 larryhl

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Posted 06 June 2007 - 08:38 PM

could you not just fold one cord in half then make note of the middle, then fold that half in the middle and make note of the quarter, then light and wait for that quarter to burn?



i believe you are expecting for the quarter to burn for 15 minutes. however, due to the nature of the cords not burning at regular speeds in the middle of the cords, folding the cord into quarters won't necessarily give you a time of 15 minutes. the reason we can safely measure half an hour is because we know that the entire cords will burn for 1 hour, so if you light it from the ends, the cord will only last 30 minutes. however, if you start burning it from somewhere that isn't an end, you will get variable times.
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#8 coolastro1016

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Posted 11 June 2007 - 02:20 AM

The first guy has the right idea. like me
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#9 TheBoss

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Posted 06 August 2007 - 09:58 PM

I believe this solution is also possible .. Please comment ...

1. Ignite the first cord at the two ends and somewhere approximately in the middle so that the fire from the middle propagates towards the two ends. It should take 15 minutes for this first cord to burn out completely (There would be two pieces of the first igniter cord since it was ignited at the middle as well - EVERY LAST BIT should burn out)

2. Ignite at the second cord at the two ends as soon as the first cord "completely" burns out. This would take another 30 minutes.
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#10 KbCb

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Posted 09 August 2007 - 02:12 PM

All of this seems fine to me except one thing... Didn't the original question request timing 45 minutes, rather than the 30 everyone is talking about?
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