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Twenty metal blocks (Posted on 2005-06-02) Difficulty: 3 of 5
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?

See The Solution Submitted by pcbouhid    
Rating: 3.5000 (2 votes)

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re(2): between... | Comment 7 of 18 |
(In reply to re: between... by Cory Taylor)

I agree that "some" includes the possibility of "all".  So I guess in this instance, since we're assured that some are aluminum, if all are found to be equal in weight, then they are all aluminum, and none (the "rest") are duraluminum.  So only 1 possibility should have been subtracted (all duraluminum--no aluminum) rather than 2.  It doesn't make much difference though--the minimum theoretically possible for an answer is still 13, though we don't know if that's actually achievable.
  Posted by Charlie on 2005-06-04 04:08:35

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