Question of the Day #289 (29/6/10)

[Kazza]

In front of you, there are 9 coins. They all look absolutely identical, but one of the coins is fake. However, you know that the fake coin is lighter than the rest, and in front of you is a balance scale. What is the least number of weighings you can use to find the counterfeit coin?


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[Popeye70]

PM sent. :cheer:

 

[06fz6green]

Sent a pm at 7:05

 

[Humperdinkel]

PM sended from QLD to WA :thumbup: , hope its faster to Aussie Post or im not gunna win

 

[Adirondack Jack]

Weighed my options and sent PM

 

[FizzySix]

I'll weigh in on this one.

 

[SirIsaac]

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[ELIZABETH]

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[y@m@h@]

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[craig007]

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[Kazza]

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[BusyWeb]

sent a guess...

 

[JustinID]

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[Fz6Sa]

Pm Sent :cheer:

 

[Kazza]

In front of you, there are 9 coins. They all look absolutely identical, but one of the coins is fake. However, you know that the fake coin is lighter than the rest, and in front of you is a balance scale. What is the least number of weighings you can use to find the counterfeit coin?

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.


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