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Pastimes : Brain Teasers.
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To: stoplossboy who wrote (87)2/4/1997 4:04:00 AM
From: space cadet  Read Replies (1) of 136
 
I didn't check the earlier postings to see if this old chestnut has already been brought up. There are 12 balls identical in all details except that 1 ball is heavier or lighter than the other 11, which all weigh the same. You are given a balance scale (the kind with two pans on each end, when the objects on each end weigh the same the scale balances). You are allowed to use the balance scale 3 times. At the end of the third weighing you must tell me,
a) which ball is the different one, and
b) is it heavier or lighter than the other 11.

This isn't the kind of problem you can do in your head. You'll need to have a paper and a pencil and try out some combinations. It's a little tricky, but it's basically a problem in combinatorics.

Want a hint-then keep reading. On a balance scale the heavier side of the scale will go down, and the lighter side will go up. Good luck.
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