coins 1,2,3&4 on one side 5,6,7&8 on other side
if balanced these 8 are even.
take 3 good coins on one side, take coin 9,10,&11 on other side
if balanced,coin 12 is bad, compare to any coin to see if heavy or lighter!
if 9,10&11 are not even and weigh more or less, put 9 on one side of scale and 10 on other side of scale, if even 11 is bad, if not then you know if it’s 9 or 10
if 1,2,3&4 goes to one side then 9,10,11&12 are good
take coin 1,5&6 on one side and coins 2,7&8 on other
if even 3 or 4 bad weigh against each other
if it goes down on one side we know one coin is light on side a or 1 of 2 is heavy on side b
weigh 2 coins that might be heavy if even 3rd coin is light if not which ever is heavy is the bad coin