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Bomb fuse puzzle

  • 06-06-2008 3:47pm
    #1
    boffin54321
    Registered Users Posts: 80 ✭✭


    2 fuses - both will burn for exactly 1 hour - however they are of differing thickness and will not burn at a uniform rate.

    How do you measure exactly 45 mins?


Comments

  • #2
    shrapnel222
    Registered Users Posts: 3,118 ✭✭✭shrapnel222


    light both fuses at the same time except light one at both ends. as soon as the double lit fuse is gone, light the second end of the fuse still burning. hey presto!:pac:


  • #3
    elmolesto
    Registered Users Posts: 5,721 ✭✭✭elmolesto


    shrapnel222 wrote: »
    light both fuses at the same time except light one at both ends. as soon as the double lit fuse is gone, light the second end of the fuse still burning. hey presto!:pac:

    Well done


  • #4
    Sherifu
    Registered Users Posts: 17,727 ✭✭✭✭Sherifu


    Very simple when you know how. Nice puzzle.


  • #5
    ojewriej
    Closed Accounts Posts: 1,468 ✭✭✭ojewriej


    I'm a bit dopey this morning, so i'm probably missing something - but I don't think this puzzle makes sense.

    If I understand it correctly, the answer is based on an assumption that fuse one lit on both sides will burn for 30 mins. But since in the riddle it's pointed out that the fuses won't burn at the unifformed rate, can you make this assumption?


  • #6
    Dufresne
    Registered Users Posts: 201 ✭✭Dufresne


    I agree with last poster - thought I was the only one who didn't get it


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  • #7
    shrapnel222
    Registered Users Posts: 3,118 ✭✭✭shrapnel222


    ojewriej wrote: »
    I'm a bit dopey this morning, so i'm probably missing something - but I don't think this puzzle makes sense.

    If I understand it correctly, the answer is based on an assumption that fuse one lit on both sides will burn for 30 mins. But since in the riddle it's pointed out that the fuses won't burn at the unifformed rate, can you make this assumption?

    the fuse won't burn at a uniform rate, which means you can't just cut it in half and burn one half for 30 minutes. let's say the piece of string is a metre long, it might burn 90 cms in half an hour, which means the last 10cms will burn in half an hour also, so by lighting it at both ends, you can be certain that whenever the string finishes burning, half an hour has elapsed.


  • #8
    boffin54321
    Registered Users Posts: 80 ✭✭boffin54321


    Hi Guys - shrapnell is right. By lighting both sides of fuse at same time, you're guaranted to measure 30mins/15mins. Well done


  • #9
    ojewriej
    Closed Accounts Posts: 1,468 ✭✭✭ojewriej


    Ok, but if the fuse doesn't burn evenly, how do you measure out a piece which will burn for exactly one hour?


  • #10
    5h4mr0(k
    Registered Users Posts: 520 ✭✭✭5h4mr0(k


    ojewriej wrote: »
    Ok, but if the fuse doesn't burn evenly, how do you measure out a piece which will burn for exactly one hour?

    You don't.
    Fuse 1 burns for one hour and fuse two burns for one hour.


  • #11
    ojewriej
    Closed Accounts Posts: 1,468 ✭✭✭ojewriej


    5h4mr0(k wrote: »
    You don't.
    Fuse 1 burns for one hour and fuse two burns for one hour.

    But how can you know that if they don't burn evenly?


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  • #12
    Irish_Nomad
    Closed Accounts Posts: 88 ✭✭Irish_Nomad


    Don't get hung up on the length just concentrate on the time. You have been given the fuses and are told that they will burn for 1 hour. You don't need to cut or measure anything.

    If the fuse takes 1 hour to burn then it must consist of 2 parts that will burn for 30 minutes each (halves in terms of time but not necessarily length). By lighting the fuse at both ends it reaches the middle (timewise not length) in 30 minutes. The same logic can then be applied to the second fuse and halve the remaining 30 minutes.


  • #13
    ojewriej
    Closed Accounts Posts: 1,468 ✭✭✭ojewriej


    Ah, fair enough, to be honest I'm just being petty because this bugs me.

    I heard this riddle before, and had similar conversation, couldn't prove my point ther either. I was hoping that maybe here someone with a better grasp of maths would be able to help me out. But since it doesn't look like this is going to happen, I stand corrected.


  • #14
    5h4mr0(k
    Registered Users Posts: 520 ✭✭✭5h4mr0(k


    ojewriej wrote: »
    But how can you know that if they don't burn evenly?

    Because it says "both will burn for exactly 1 hour" in the original question. It's a given.


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