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 What's the problem? (and a spoiler)
by Steve Herman)
The problem is to identify any one coin. It does not need to be a specific coin. Broll's solution is correct.
I don't think it is possible to choose and identify a specific coin in just three weighings.