Best Answer plainglazed, 09 February 2014 - 03:36 AM

Spoiler for three weighings...

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...if the normal unit weight is known:

1) weigh seven coins

if they weigh seven units the odd coin must be one of the other five coins

2) weigh three of those five coins

if they weigh three units the odd coin must be one of the last two.

3) weigh one of those two coins to see which is the odd.

if they weigh more (or less) than three units one of the three must be the odd coin.

3) of those three; flip one, put one on edge and leave one as is to find the odd coin.

if they weigh more (or less) than seven units the odd coin must be one of those seven.

2) weigh four of those coins flipping two of the four over.

if the four coins are more (or less) than four units, the odd coin must be one of the two not flipped.

if the four coins are less (or more) than four units, the odd coin must be one of the two flipped.

3) weigh one of those two coins to see which is odd.

if the four coins weigh four units the odd coin must be one of the three of the seven not weighed a second time.

3) of those three; flip one, put one on edge and leave one as is to find the odd coin.

1) weigh seven coins

if they weigh seven units the odd coin must be one of the other five coins

2) weigh three of those five coins

if they weigh three units the odd coin must be one of the last two.

3) weigh one of those two coins to see which is the odd.

if they weigh more (or less) than three units one of the three must be the odd coin.

3) of those three; flip one, put one on edge and leave one as is to find the odd coin.

if they weigh more (or less) than seven units the odd coin must be one of those seven.

2) weigh four of those coins flipping two of the four over.

if the four coins are more (or less) than four units, the odd coin must be one of the two not flipped.

if the four coins are less (or more) than four units, the odd coin must be one of the two flipped.

3) weigh one of those two coins to see which is odd.

if the four coins weigh four units the odd coin must be one of the three of the seven not weighed a second time.

3) of those three; flip one, put one on edge and leave one as is to find the odd coin.