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Fake coins

  1. May 5, 2005 #1


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    there are alot of these fake coin/find it in three weighings etc problems. i have a few that i like, and maybe someone here can add some that they like/know? just an idea :)

    anyway this is a funny one, if you have 6 coins and 2 of them are fake, the question is can you determine (in just *3* weighings) which coins they are assuming they are lighter than the other 4 real coins?

    i have trouble spelling many things :yuck:

    oh and the scale tells you only which side is heavier. (duh) :biggrin:
  2. jcsd
  3. May 5, 2005 #2
    Code (Text):

    Number the coins 1-6
    Weigh: 1&2 vs 3&4
    If scale balances:
         Weigh: 1&3 vs 2&4
         If scale balances:
                Weigh: 1&4 vs 2&3
                If scale balances: 5 & 6 are the phonies
                otherwise the lighter side contains both phonies
         otherwise the lighter side contains both phonies
    otherwise, name the lighter-side coins L1 and L2.
         Weigh: L1 vs 5
         If scale balances:
             Weigh: L1 vs 6 (scale cannot balance)
             If L1 is lighter, then L1 and 5 are the phonies
             If 6 is lighter than L2 and 6 are the phonies
         If 5 is lighter, it is a phony and L2 is the other phony.
         If L1 is lighter:
             Weigh: L1 vs L2
             If the scale balances, L1 and L2 are the phonies
             Otherwise 6 and L1 are the phonies
  4. May 5, 2005 #3
    That's interesting and i believe correct but wouldn't it be easier to split the coins into two halves. Three and three, if they balance weigh two from each half and from that you can figure out which coins are fake, if the originals don't balance then you only have to weigh once more to determine the fake coins.
  5. May 5, 2005 #4
    I don't think that would work. After they balance the first time and you weigh 2 from each half, say that the 2 balance. Now either the lighter coins are both contained in the two pairs you weighed, or both are not. You can't determine where they are with only the one remaining weighing.
  6. May 5, 2005 #5
    if they balance a second time then the coin which wasn't weighed is the lighter one and you can move on to weighing the other half.
  7. May 5, 2005 #6
    Because there are only two fake coins if they balance the first time it means there is a fake coin in each half. If it doesn't it means that the lighter side contains both fake coins, hence you only have to weigh one more time after that.
  8. May 5, 2005 #7
    Here's an interesting question which I do not know the answer to. If you have n coins total and r fake ones out of those n, how many weighings do you need to separate the fakes coins from the genuine coins?
  9. May 5, 2005 #8
    For that question, that's assuming that you can only weigh equal #'s of coins. i.e. no determining the weight of a fake coin by trying, say, 4 known-real coins vs 5 known-fake coins.
  10. May 5, 2005 #9
    I'm guessing there are many different solutions to the same question depending on how many fake ones there are and if they are an odd or even number of them. There is an algorithm that works for every combination (weighing half and half until you eliminate each possibility), but to get an efficient process you'd need to know n and r. Therefore, depending on what process you use and luck you may have to weigh many times or fewer in certain cases. In short i don't know either.
  11. May 5, 2005 #10
    There were 2 that were not weighed in the second weighing in your scheme.
    Your scheme is:
    Code (Text):

    Weigh 1&2&3 vs 4&5&6
    If the scale balances
       Weigh 1&2 vs 4&5
       If the scale balances
           You're out of luck.
  12. May 5, 2005 #11
    Code (Text):

    Weigh first half(1&2&3) vs second half(4&5&6)
    If the scale balances
       For first half:
         Weigh 1 vs 3
         If the scale balances
             2 is lighter.
         otherwise you have found either 1 or 3 to be fake.
       For second half:
         Weigh 4 vs 6
         If the scale balances
             5 is lighter.
         otherwise you have found either 4 or 6 to be fake.
    If the scales don't balance set aside the lighter side three coins name them 1,2,3
        Weigh 1 vs 3
         If the scale balances
             they are the fake coins.
         otherwise you have found either 1 or 3 to be fake and 2 is fake by elimination.
  13. May 5, 2005 #12
    Okay, that works. Your original description was unclear.
  14. May 8, 2005 #13
    by the way, is fake coins 'always' lighter.
    because i once given this kind of puzzle by a professor and he use stone instead of coins. and he also added that the weight is 'diffrent' without mentioning wether the 'fake' is lighter of heavier. n i it still ringing in my head until now.
  15. May 8, 2005 #14
    Do you remember how many fakes and reals there were and how many weighs you had?
  16. May 8, 2005 #15


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    wait a second: whay if you have say n coins, and all n of them are fake. how on earth are you going to find the n fake coins if you dont know how many there are (!)

    theres also a funny one thats slightly different,

    you have 10 sacks of coins. each sack contains *many* coins. you know that one sack is entirely fake, and that the difference in weight between that sack and the others is exactly 1 unit. in one weighing using a scale that tells you the exact difference in weight determine the bag containing the fake coins.

    this one is funny in that the last part is a little confusing and can be done in the easy and harder way(s)
  17. May 8, 2005 #16
    I've heard this one before.
  18. May 8, 2005 #17


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    Science Advisor
    Homework Helper

    Not too hard.

    Weigh all the coins. If the scale tips both the light coins are on the same side, and all you have to do is eliminate the heavy one. You can do this in one more weighing because if they don't balance you know which is heavier and if they do balance the one you didn't weigh is heavier.

    If the scale balances, do the one weighing for each side to find out which are the lightest.

    Easy stuff!
  19. May 8, 2005 #18
    No, the question is to find the counterfeit bag in one weighing.
  20. May 10, 2005 #19


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    sorry ive been sortof delinquent on this post. anyway i didnt hear any answers to it and it has been awhile so ill just say the solution:

    you take 1 coin from bag 1, 2 from bag 2, 3 from 3 etc and put them on one side of the scale. then take 45 (sum of 1 - 10) from *any* bag and put it on the other side. clearly if the 45 same coins are real than the weight difference corresponds to the number of the bag with fake coins. if theyre fake you'l get a number that much bigger than 10 and you should know that the 45 are fake and either know which bag you took them from or go through some easy math to calculate which one it is.

    sadly i cant recal a really hard one, maybe someone here knows it...? its something to do with 2 weighings and like 10 coins and one is fake (lighter)... anyone?

    hope im not borring anyone here, i think its cool personaly...
  21. May 11, 2005 #20
    The closest I can come is: 9 coins, 1 is light, find the light one in 2 weighings on a balance scale. But it isn't really hard. The problem scales in the sense that you can start with 27 coins and find the light one in 3 weighings, etc.
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