Todays Brain Teaser #35 (11JUL2013)

[vpnavy]

Today's Brain Teaser #35 (11JUL2013)

You have 9 coins. They all appear to be identical but one of the coins is fake! Now, you know that the fake coin is lighter than the rest of the coins. Fortunately, you have a balance scale.

What is the least number of weighings you can use to find the counterfeit coin?

 

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[au-artifax]

Ok. If you are lucky... Just one. But there is a 100% chance of knowing the fake coin by the third weighing.


PS: GO NAVY!!! (class 4285 Batt I)

 

[vpnavy]

Ok. If you are lucky... Just one...

 

[GMD52]

3, 1 diff, and 2 to verify same weight

 

[Uncle Jeff]

3, which is also the same amount of licks it takes to get to the center of a lollipop

 

[vpnavy]

Focus gang - don't make this difficult - Question: What is the least number of weighings? Is it 1, 2, 3, 4, 5, 6, 7, 8 or 9?

 

[GMD52]

Let's try again.......If you already know the fake coin is lighter, you shouldn't have to weigh any?

 

[Mackaydon]

2; weigh 3 on each side first, if they balance the light coin is still in your hand. then weigh two of them. If they balance, it's the one you're still holding; otherwise, the scale will reflect the lighter of the two coins. If the first 'balance' test shows an in-balance, the lighter coin will be in the threesome that rose. Then weigh only two of them to determine if one is lighter OR you're holding the lightest.
Don....

 

[Crispin]

zero, they "All appear to be identical but one" the one that does not appear identical is the fake coin.

 

[Uncle Jeff]

zero, they "All appear to be identical but one" the one that does not appear identical is the fake coin.

Crud when you take out the bold word it is so obvious .. Grrrrrrrr

 

[Uncle Jeff]

Although in VPNavy's clue he states 1 through 9 and NO ZERO ...

 

[Red James Cash]

I do believe it is 8 times

 

[au-artifax]

Hold the presses..... Who says you have to use the scale? There are other ways to tell. If you were free to NOT use the scale then the answer would be ZERO, ZILCH, NADA, NONE

 

[au-artifax]

AND.... if we are using the scale the answer is 1 (one), and strike the "if you are lucky" part.
You see...if you put the coins on the scale one at a time alternating which side you put the coin on each time, you will know at some point during that first weighing.
TAADAAAA!!!!!

 

[garren]

I think its 5...

Divide the coins in groups of three and weigh each group. One will be lighter. Now you know your bad coin is in the lighter group. Weigh two coins from that group. If the two coins you choose weigh the same then the one you didnt weigh is the fake. If you weigh two coins and one is lighter then its the bad coin. Either way its only five weighs.

 

[vpnavy]

You have 9 coins. They all appear to be identical but one of the coins is fake! Now, you know that the fake coin is lighter than the rest of the coins. Fortunately, you have a balance scale.

What is the least number of weighings you can use to find the counterfeit coin?

3, 1 diff, and 2 to verify same weight

Correct!

- now, after re-reading everyone's answers - it appears that Mackaydon had the correct answer, GMD52 slid into the correct answer - and yet au-artifax indicated by the third weighing which meant got it - I am so confused by you guys - so, this is my final answer to this question and both will be declared winners - no discussion - I am the boss!

The answer is 2.

First, divide the coins into 3 equal piles.

Place a pile on each side of the scale leaving the remaining pile of 3 coins off the scale.

If the scale doesn't tip you know that the 6 coins on the scale are legitimate and the counterfeit is in the pile in front of you.

If the scale does tip, you know the counterfeit is in the pile on the side of the scale that raised up.

Either way, put the 6 legitimate coins aside.

Having only 3 coins left put a coin on each side of the scale leaving the third in front of you.

The same process of elimination will find the counterfeit coin.

Stay tuned for tomorrow's!
==============================
BRAIN TEASER WINNERS
==================
(35 - Jul 11, 2013) GMD52 (2), au-artifax and Mackaydon (4)
(34 - Jul 10, 2013) rickv14623
(33 - Jul 09, 2013) Crispin (7)
(32 - Jul 08, 2013) Red James cash (3)
(31 - Jul 07, 2013) jman774 (5)
(30 - Jul 06, 2013) Uncle Jeff (2)
(29 - Jul 05, 2013) garren (2)
(28 - Jul 04, 2013) jman774 (4)
(27 - Jul 03, 2013) GMD52 (1)
(26 - Jul 02, 2013) Red James cash (2)
(25 - Jul 01, 2013) Uncle Jeff (1)
(24 - Jun 30, 2013) Mackaydon (3)
(23 - Jun 29, 2013) Mackaydon (2)
(22 - Jun 28, 2013) andre1002 (2)
(21 - Jun 27, 2013) jman774 (3)
(20 - Jun 26, 2013) andre1002 (1)
(19 - Jun 25, 2013) Crispin (6)
(18 - Jun 24, 2013) jman774 (2)
(17 - Jun 23, 2013) pong12211
(16 - Jun 22, 2013) jman774 (1)
(15 - Jun 21, 2013) Crispin (5)
(14 - Jun 20, 2013) Crispin (4)
(13 - Jun 19, 2013) BrisJoe (3)
(12 - Jun 18, 2013) BrisJoe (2)
(11 - Jun 17, 2013) Crispin (3)
(10 - Jun 16, 2013) BrisJoe (1)
(09 - Jun 15, 2013) Mackaydon (1)
(08 - Jun 14, 2013) Crispin (2)
(07 - Jun 13, 2013) No Winner
(06 - Jun 12, 2013) garren (1)
(05 - Jun 11, 2013) Red James cash (1)
(04 - Jun 10, 2013) Backbacon
(03 - Jun 09, 2013) No Winner
(02 - Jun 08, 2013) frogginfish
(01 - Jun 07, 2013) Crispin (1)
(#) = Number of Wins