The dork tries again...PUZZLE

# Thread: The dork tries again...PUZZLE

1. Senior Member
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Dec 1969
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96,118

## The dork tries again...PUZZLE

Let&#039;s do that coin puzzle all over again.<BR><BR>You have several rolls of coins.<BR><BR>One roll contains counterfeit coins.<BR><BR>All the other rolls contain genuine coins.<BR><BR>A counterfeit coin weighs 0.1 gram less than a genuine coin. [Or 0.1 gram more, or 0.1035 grams less, or... the number, per se, doesn&#039;t matter.]<BR><BR>You have a precision scale that can weigh accurately from 0.1 grams to hundreds of grams.<BR><BR>What is the minimum number of weighings you need to find which roll is the counterfeit one, NO MATTER WHICH ROLL is counterfeit?<BR><BR>HINT: The answer is one. <BR><BR>Now explain how.<BR><BR>Oh, and tell what the limitation of the method is.<BR><BR><BR>

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## 1, 2, 3 testing...

Weight by pairs on the minimal case (when a counterfeit exists) 1st weighting is different to second weighting on the second attemp<BR><BR>x = real roll of coins<BR>y = counterfeit roll<BR><BR>x + x &#060;&#062; x + y<BR>or <BR>x + y &#060;&#062; x + x<BR>or<BR>x + x &#060;&#062; x + 1/y<BR>or <BR>x + 1/y &#060;&#062; x + x

3. Senior Member
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## One

and a phone call to the Secret Service to report phony money... let them figure it out<BR><BR>(I&#039;ll leave out the part that the Secret Service tried recruiting me at one point and did ask me a bunch of questions relating to counterfeiting) ;-)

4. Senior Member
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## Keep trying...

I already gave you the minimum number of weighings: ONE<BR><BR>Now you explain HOW to do it in one weighing.<BR><BR>

5. Senior Member
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## None

You take them all to WalMart and buy Starwars DVD

6. Senior Member
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## RE: The dork tries again...PUZZLE

Sigh...<BR><BR>Ok. Place all coins on the scale. Since one of them throws off into a &#039;float&#039; number, place all of them on at once, and remove one at a time. If all 10 weigh 10.01 whatever ounces/grams, as soon as the &#039;float&#039; part happens, that&#039;s the counterfeit roll.

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## LOL! A nice cheat...

...But I&#039;ll only give you a C+ for it, since technically each time you remove a coin you are making another weighing.<BR><BR><BR><BR>

8. Senior Member
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## ok

seven rolls<BR>you put three rolls on each side -- <BR><BR>If they balance then the one that was not weighted is the counterfeit roll

9. Senior Member
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## RE: LOL! A nice cheat...

Ugh... true.<BR><BR>I guess that&#039;s why I married a model and didn&#039;t spend my time dinking with puzzles like these ;-)<BR><BR>Although, that is construed as one weighing, in that all coins are placed on the scale at once. Looking at the scale for weight shouldn&#039;t constitute a second and subsequent weighing...

10. Senior Member
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## And what if it doesn't balance?

Then you have to do a second weighing...

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