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The Rings (Posted on 2002-09-20) Difficulty: 3 of 5
Mary has an unusual ring. It is made up of a set of three interlocking ringlets, each set with a large gemstone, a diamond, a ruby, and a sapphire. It is very valuable and the insurance company insists it be kept locked in their vault, except on those occassions where Mary is wearing it. Because of this, Mary had a copy made, which she wears on lesser occassions.

Today she attended an affair in which she was able to convince the insurance company that she needed to wear the original. Half an hour after returning home the insurance rep called to let her know he was on his way to pick up the ring.

Mary then realized that she'd absent-mindedly taken off the ring and put it in her jewelry box, where she keeps the copy.

When it's not on her finger, the ring separates into its componants, so Mary was looking at six nearly identical ringlets, two with blue stones, two with red, and two with white.

She needs to separate the genuine ringlets from the copies. She knows that each of the ringlets in each set weighs the same.(That is the genuine ringlets each weigh the same, and the copies each weigh the same.) She also knows that a copy weighs less than its original.

The only scale she has that is delicate enough to properly weigh the ringlets is a small balance scale she uses to measure headache powders and sleeping draughts (she can't swallow pills), but the weights tha she uses for her medicine are of an order too small for the ringlets. She will have to weigh them against each other. She could do it in three weighings by trying each against its counterpart, but she is certain to be "caught" before she finishes, and either her insurance will go up, or the company will be "forced" to not allow her to wear the ring any more.

Is there a way to separate the six ringlets in two weighings?

  Submitted by TomM    
Rating: 2.7500 (4 votes)
Solution: (Hide)
Yes. One possible sequence:

First weighing: put a white and a red in one pan, a white and a blue in the other.

Case I (they balance): Since you don't have four genuine or four copies, you must have two of each. Switch the whites, so that the copies are together in one pan and the orginals in the other to determine which is which. The unweighed ringlets can each only go in one set.

Case 2 (they don't balance):

In this case, you have either three copies and one genuine, or three genuine and one copy, or you have two heavies vs two lights. Either way, you've isentified the whites.

Carefully set them aside and note which color the genuine white was paired with. Put the red and blue you weighed in one pan and the other two in the other pan. If they do not balance, the heavier pan has two originals and the lighter, twoo copies. If they balance, the ringlet that was partnered with the genuine white was also genuine, and it's new parner is a copy, and the other two can be worked out from there.

Comments: ( You must be logged in to post comments.)
  Subject Author Date
Some ThoughtsTwo weighings take longerlucky2002-09-20 12:04:22
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