Maths problems (interesting ones)

[Alan Anderson]

Originally Posted by diobsidian

I hate to break it to you after all your work, but there are 12 coins, and lettering them A through I is only 9.

Whoops.

I still have a 3/4 chance of finding the coin and identifying it as heavy or light in three weighings if I simply ignore the last three, and it will only take one extra weighing in that 1/4 case, but that obviously doesn't meet the requirements of the problem, does it?

I know how to do the harder problem in theory, choosing appropriate groupings of coins to get the equivalent of three equations with three unknowns, but I lack the time to figure out the details today.