I have two sets of 8 coins. In one set the coins weigh 30g each, in the other set the coins weigh 31g each.
Unfortunately they got mixed together in one big pile of 16 coins. I want to identify one coin. It can be from either set.

(Easy) Using a balance scale, identify a coin in four weighings.

(Hard) Identify a coin in just three weighings.

(In reply to re: Short solution. by Jer)

You are right. The 2v2 part is a dead end, left over from something I did earlier, though the odd 4 may still have a role to play.

6v6 is still right, with a slight riff on the theme if the 3v3 balances on the second weighing.

Thanks for pointing this out; you had me worried there for a while, particularly as I'd been told my solution was correct!

 

Edited on January 12, 2016, 12:07 am

Posted by broll on 2016-01-11 23:08:08