The question as it appears in the 01/23 edition of Specs & Techs from GlobalSpec:
You have twelve identicallooking coins; one is counterfeit. The only measurable difference is that the bad coin weighs either more or less than the others. Using a balance scale, determine which coin is bad and whether too light or too heavy. You are limited to three weighings on the scale. (Note: there are 24 possible outcomes; your method must account for all 24 cases.)
This question was submitted by our very own AstroNut!
Update (01/30/07 9:04 AM): And the Answer is....
1) Put coins 14 on the right, 58 on the left, and leave 912 off.
 IF 1) BALANCES: 18 are good.
 2) Put coins 13 on the right, 911 on the left.
 IF 2) BALANCES: Coin 12 is bad.
 3) Weigh 12 against 1 to see whether 12 is light or heavy. Done!
 IF 2) GOES LEFT SIDE DOWN: The bad coin  9, 10, or 11  is heavy.
 3) Put 9 on the right, 10 on the left.
 IF 3) BALANCES: Coin 11 is bad (heavy).
 IF 3) DOESN'T BALANCE: The heavier coin  9 or 10  is bad.
 IF 1) GOES LEFT SIDE DOWN: 912 are good.
 2) Put coins 1, 5, and 6 on the right; 2, 7, and 8 on the left.
 IF 2) BALANCES: Coin 3 or 4 is bad (light).
 3) Put 3 on the left, 4 on the right. The lighter coin  3 or 4  is bad.
 IF 2) GOES LEFT SIDE DOWN: EITHER 1 is light OR 7 or 8 is heavy.
 3) Put 7 on the left, 8 on the right.
 IF 3) BALANCES: Coin 1 is bad (light).
 IF 3) DOESN'T BALANCE: The heavier coin  7 or 8  is bad.
 IF 2) GOES RIGHT SIDE DOWN: follow similar procedure with 2, 5, and 6.
 IF 1) GOES RIGHT SIDE DOWN: follow the same procedure, reversing light & heavy.

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