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a simple brain teaser

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2

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    #32
    Ficheall
    Registered Users Posts: 9,018 ✭✭✭Ficheall


    kraggy wrote: »
    Here's one while we're at it.

    You have

    a. A weighing balance
    b. 9 balls

    8 of the balls are the same weight, one is a different weight.

    Question: What is the least number of times you can think of using the weighing balance to determine which ball is of a different weight?

    3

    Do I get a prize for beating bluewolf? :)


    Edit: I presume it's a lower bound on the maximum number of necessary weighings to determine which ball is of a different weight that you are looking for.


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    #33
    kraggy
    Closed Accounts Posts: 6,414 ✭✭✭kraggy


    Ye can't just guess a shaggin number! Ye have to explain yourselves!!


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    #34
    coffee_cake
    Moderators, Category Moderators, Politics Moderators, Recreation & Hobbies Moderators, Society & Culture Moderators Posts: 81,310 CMod ✭✭✭✭coffee_cake


    Ficheall wrote: »
    3

    Do I get a prize for beating bluewolf? :)

    Hey, that was one of my answers :pac::pac::pac:


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    #35
    sock puppet
    Registered Users Posts: 4,586 ✭✭✭sock puppet


    It's 2.


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    #36
    Ficheall
    Registered Users Posts: 9,018 ✭✭✭Ficheall


    kraggy wrote: »
    Ye can't just guess a shaggin number! Ye have to explain yourselves!!

    And spoil it for others? Don't be daft.
    It's a gorgeous problem.


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    #37
    Micky Dolenz
    Registered Users Posts: 44,080 ✭✭✭✭Micky Dolenz


    3, has to be.

    Give us something hard, oh er matron.


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    #38
    Skid X
    Registered Users Posts: 21,444 ✭✭✭✭Skid X


    8 out of 10 Cats does Countdown is on Channel 4 at 9pm.

    Featuring Rachel Riley :)


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    #39
    kraggy
    Closed Accounts Posts: 6,414 ✭✭✭kraggy


    Micky Dolenz wrote: »
    3, has to be.

    Give us something hard, oh er matron.

    nope.


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    #40
    coffee_cake
    Moderators, Category Moderators, Politics Moderators, Recreation & Hobbies Moderators, Society & Culture Moderators Posts: 81,310 CMod ✭✭✭✭coffee_cake


    Vowel please, carol


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    #41
    kraggy
    Closed Accounts Posts: 6,414 ✭✭✭kraggy


    bluewolf wrote: »
    Vowel please, carol

    OOOOOoooooo Carol...


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    #42
    Ficheall
    Registered Users Posts: 9,018 ✭✭✭Ficheall


    kraggy wrote: »
    nope.
    What do you mean, 'nope'? It is 3.


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    #43
    kraggy
    Closed Accounts Posts: 6,414 ✭✭✭kraggy


    Ficheall wrote: »
    What do you mean, 'nope'? It is 3.

    It's not.


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    #44
    sock puppet
    Registered Users Posts: 4,586 ✭✭✭sock puppet


    sock puppet wrote: »
    It's 2.
    kraggy wrote: »
    Ye can't just guess a shaggin number! Ye have to explain yourselves!!

    Fine. Weigh 6 and leave 3 to the side. If they don't balance weigh 2 of the 3 on the heavy side and find the heavy one. If they do balance weigh 2 of the 3 remaining balls.

    I've rather gone off these problems since employers ask them in interviews.


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    #45
    coffee_cake
    Moderators, Category Moderators, Politics Moderators, Recreation & Hobbies Moderators, Society & Culture Moderators Posts: 81,310 CMod ✭✭✭✭coffee_cake


    Ficheall wrote: »
    Unless it's a trick question, or you haven't given us enough information, such as there being a limit of the number of balls that fit on the balance, then it is.

    You didn't beat me! Hand back the prize!


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    #46
    kraggy
    Closed Accounts Posts: 6,414 ✭✭✭kraggy


    sock puppet wrote: »
    Fine. Weigh 6 and leave 3 to the side. If they don't balance weigh 2 of the 3 on the heavy side and find the heavy one. If they do balance weigh 2 of the 3 remaining balls.

    I've rather gone off these problems since employers ask them in interviews.

    Correct :)


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    #47
    Ficheall
    Registered Users Posts: 9,018 ✭✭✭Ficheall


    kraggy wrote: »
    Correct :)

    No, it isn't.


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    #48
    kraggy
    Closed Accounts Posts: 6,414 ✭✭✭kraggy


    Ficheall wrote: »
    Unless it's a trick question, or you haven't given us enough information, such as there being a limit of the number of balls that fit on the balance, then it is.

    You only need to use the balance 2 times. As demonstrated by my lovely assistant Sock.


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    #49
    Ficheall
    Registered Users Posts: 9,018 ✭✭✭Ficheall


    kraggy wrote: »
    You only need to use the balance 2 times. As demonstrated by my lovely assistant Sock.

    No, I'm afraid you're mistaken. You might want to re-read your question.


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    #50
    kraggy
    Closed Accounts Posts: 6,414 ✭✭✭kraggy


    Ficheall wrote: »
    No, it isn't.

    Yes it is. The wording might be different to how I'd put it, but the essence is the same.


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    #51
    humbert
    Registered Users Posts: 5,238 ✭✭✭humbert


    I was trying to think how I'd do it with my electric scales :o


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    #52
    Feathers
    Registered Users Posts: 1,082 ✭✭✭Feathers


    Ficheall wrote: »
    No, it isn't.

    He's just explained it step-by-step :confused:
    Or are you saying you can do it in one go? :)


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    #53
    coffee_cake
    Moderators, Category Moderators, Politics Moderators, Recreation & Hobbies Moderators, Society & Culture Moderators Posts: 81,310 CMod ✭✭✭✭coffee_cake


    Ficheall wrote: »
    No, it isn't.

    If you pick 6 balls and weigh 3 on either side...
    one is heavier
    weigh 2 of the three. Either it's one of them, or it isn't - in which case it's the #3 you set aside

    If you weighed the 3 on either side and they balanced, weigh 2 of the remaining 3... either they balance or they don't, either way you know

    Works out to me

    Same if you flip it to "which is lighter", I suppose


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    #54
    kraggy
    Closed Accounts Posts: 6,414 ✭✭✭kraggy


    Ficheall wrote: »
    No, I'm afraid you're mistaken. You might want to re-read your question.

    Explain.


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    #55
    K-9
    Registered Users Posts: 43,311 ✭✭✭✭K-9


    5 squared (can't do the symbol thingy) = 25 + 4 = 29, 3-2 = 1, 29-1 = 28

    Mad Men's Don Draper : What you call love was invented by guys like me, to sell nylons.



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    #56
    Ficheall
    Registered Users Posts: 9,018 ✭✭✭Ficheall


    kraggy wrote: »
    Explain.

    You haven't said whether the 9th ball is heavier or lighten than the average!
    You just said it's different.

    Now, I've to meet someone for a stroll, but I'm sure you'll have worked it out by the time I get back ;)

    You can send me bluewolf's number as a prize or something :P


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    #57
    kraggy
    Closed Accounts Posts: 6,414 ✭✭✭kraggy


    I'll word it my own way.

    1. Split the balls into 3 groups called A, B and C, each group containing 3 balls.

    2. Put A and B on either side of the scales -> 1 use of the balance.

    Scenario a.

    If they weigh the same, then you know the heavy ball is in group C.
    Remove A and B from the balance.
    Place any 2 balls from C on to the balance -> 2nd use of the balance.
    If they balance, the last ball remaining is the heaviest.
    If they don't, the heavier one, is, er, the heavier one.

    Scenario b.

    A is heavier than B, then remove B and do the same with A as with C above.

    i.e. pick any 2 balls and you will determine which of the 3 balls in group A is the heaviest.


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    #58
    coffee_cake
    Moderators, Category Moderators, Politics Moderators, Recreation & Hobbies Moderators, Society & Culture Moderators Posts: 81,310 CMod ✭✭✭✭coffee_cake


    Ficheall wrote: »
    You haven't said whether the 9th ball is heavier or lighten than the average!
    You just said it's different.

    Now, I've to meet someone for a stroll, but I'm sure you'll have worked it out by the time I get back ;)

    You can send me bluewolf's number as a prize or something :P
    Feck it you're right! :P
    You win this round :mad:
    https://encrypted-tbn0.gstatic.com/images?q=tbn:ANd9GcRedR3Quc4hz7y0x3f2LToKMaSwuyL-buuv7N2tRz-0UT8rt1Km


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    #59
    K-9
    Registered Users Posts: 43,311 ✭✭✭✭K-9


    K-9 wrote: »
    5 squared (can't do the symbol thingy) = 25 + 4 = 29, 3-2 = 1, 29-1 = 28

    Oh right, you's have all moved on. :o

    Mad Men's Don Draper : What you call love was invented by guys like me, to sell nylons.



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    #60
    coffee_cake
    Moderators, Category Moderators, Politics Moderators, Recreation & Hobbies Moderators, Society & Culture Moderators Posts: 81,310 CMod ✭✭✭✭coffee_cake


    K-9 wrote: »
    Oh right, you's have all moved on. :o

    Here's a bouncy ball. Do you want to bounce the ball?
    :p


    /runs


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    #61
    K-9
    Registered Users Posts: 43,311 ✭✭✭✭K-9


    bluewolf wrote: »
    Here's a bouncy ball. Do you want to bounce the ball?
    :p


    /runs

    Look squirrel.

    Mad Men's Don Draper : What you call love was invented by guys like me, to sell nylons.



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