I have mentioned a number of times my high IQ and now I prove it. My solution can not only solve 12 coins but thirteen.
1.Weight 4 against each other, abcd vs efgh. If they are not balanced, go to 2.
2.Replace f and g by the coins you know now are safe, and swap cd with gh.
If the scales are now balanced, you know the odd one is in the one removed. If the scale remains the same, you know the odd one lies in abe. If they switch from heavy to light, you know it is in cdh.
Take cdh. Now you know cd is lighter, and h heavier, not not know which is the odd one. weigh c and d. if c is lighter, it means c is the light one. if they are balanced, h is the odd one out.
1.Weight 4 against each other, abcd vs efgh. If they are not balanced, go to 2.
2.Replace f and g by the coins you know now are safe, and swap cd with gh.
If the scales are now balanced, you know the odd one is in the one removed. If the scale remains the same, you know the odd one lies in abe. If they switch from heavy to light, you know it is in cdh.
Take cdh. Now you know cd is lighter, and h heavier, not not know which is the odd one. weigh c and d. if c is lighter, it means c is the light one. if they are balanced, h is the odd one out.
