Twenty metal blocks are of the same size and external appearance; some are aluminum, and the rest are duraluminum, which is heavier.

Using a pan balance to determine how many blocks are aluminum, what is the minimum number of weighings to be done?

(In reply to re(2): between... by Charlie)

It seems we (starting with me) have been misinterpreting the problem.  We don't need to identify which blocks are which--only determine how many of the twenty are duraluminum.  That's only 20 possibilities.  I doubt that it's possible in three weighings (which have 27 possible outcomes).

Posted by Charlie on 2005-06-05 22:31:36