Anybody know the solution?

Question

Given twelve coins that are identical in size, shape, and color, determine which single coin is heavier or lighter in weight than the others. You are supplied with a balance and must conclude your determination in three weighings.

(And you don't know if it's heavier OR lighter)

Answer 1

You weight 6 coins first, 3 coins on each side of the balance. If it is balanced, then the other 6 coins must contain the abnormal one. Otherwise, the abnormal one must be in the 6 coins weighted.

In either way, after the first try, you can be sure which 6 coins contain the abnormal one.

Now from the 6 coins containing abnormal, take 3 coins to balance with 3 normal coins. Therefore, you can be sure which 3 remaining coins must contain the abnormal one, and also you know the abnormal one is lighter or heavier.

Finally, you pick 2 out the 3 remaining coins containing the abnormal one to do balance to find the abnormal one.
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pol_dougl,
You cannot assume the abnormal coin is heavier.

Answer 2

If you are told that one coin is heavier or lighter, it is fairly easy to ascertain which one it is. As the problem is stated, you can not determine the answer in 3 moves.

Answer 3

Assume your lookin for the heavier coin ok?

1. Weigh the coins in 2 random piles of 6, select the heavier pile.
2. Weigh these 6 coins in 2 random piles of 3, select the heavier pile.
3. Weigh 2 random coins from this pile, if one is heavier thats the one you need...if they balance then the coin you're looking for is the one you didn't weigh.

Answer 4

I know how to do this but cant spend the time necessary to fully explain the method. If nobody else gives you the answer to this problem email me and I will show you how it is done.