You have 12 coins. They are completely identical except six of them weigh 24g and the other six weigh 25g. You have only a balance scale to sort them out. What is the minimum number of weighings which guarantees all the coins to be sorted?

(In reply to Simple approach by Brian Wainscott)

I like the "Master Coin" method. To reduce the maximum weighings from 10 to 9 (thereby tying Charlie's method) you can weigh two of the final 3 "unknown" coins on the 9th weighing. If they're equal, they both belong to whichever category you know 4 of. If they're unequal, then they're one each, and you know which is which, and you know what the 12th coin must be.

There's no need to weigh any of the final 3 coins against the Master Coin.

Cheers! :^)

Posted by John on 2004-01-19 17:52:38