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Insert Coin Here


Best Answer k-man, 17 January 2013 - 05:02 PM

Spoiler for Solution
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8 replies to this topic

#1 TimeSpaceLightForce

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Posted 17 January 2013 - 03:11 AM

Your light and heavy cheat coins weigh like 2 proper coins
but looks the same.They always hit the jackpot in any
slot machines.  Drowsy, you accidentally used them on
a vending machine along with 2 other proper coins.
However the coins malfunctioned the machine causing it
to dispense hot cup of coffee with the inserted coin inside.
To test weights, you may insert a coin using the beam-
balance machine once.
 


           \_/   \_/    \_/   \_/      


___________           __________
        |_________________|
                ____/\____     
               /              --  \
              [----------------]    o o


A) With 2 spare coins in pocket,how can you identify the cheat coins ?
B) How many spare coins is needed if the first trial is balanced ?


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#2 Prime

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Posted 17 January 2013 - 04:36 AM

I assume the vending machine returned all 4 deposited coins; and the balance takes one coin for each use.

 

Spoiler for Find cheating pair

 

Good casinos give you complementary drinks when you gamble. There was no need to use vending machine for coffee.


Edited by Prime, 17 January 2013 - 04:37 AM.

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Past prime, actually.


#3 TimeSpaceLightForce

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Posted 17 January 2013 - 06:21 AM

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#4 BobbyGo

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Posted 17 January 2013 - 04:08 PM

Spoiler for How about...

Edited by BobbyGo, 17 January 2013 - 04:10 PM.

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#5 TimeSpaceLightForce

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Posted 17 January 2013 - 04:42 PM

Spoiler for How about...
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#6 k-man

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Posted 17 January 2013 - 05:02 PM   Best Answer

Spoiler for Solution

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#7 Prime

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Posted 17 January 2013 - 08:20 PM

Spoiler for

I wouldn’t classify winning with regular coins as “problem”. Just take the winnings home and measure the remaining two coins at leisure using your own precision scale.

 

But if you insist on using the coin operated balance to the end...

Spoiler for the weighing sequence


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Past prime, actually.


#8 TimeSpaceLightForce

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Posted 18 January 2013 - 06:38 AM

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#9 k-man

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Posted 18 January 2013 - 04:20 PM

Spoiler for

 

Nice solution using the Archimedes' principle, TSLF. However, from the OP it wasn't clear that you actually had 4 cups of coffee to work with. I thought you only had one cup, which would preclude such a solution. Nice puzzle nonetheless.


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