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Masters of Logic Puzzles III. (stamps)


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#1 rookie1ja

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Posted 30 March 2007 - 04:17 PM

Masters of Logic Puzzles III (stamps) - Back to the Logic Puzzles
Try this. The Grand Master takes a set of 8 stamps, 4 red and 4 green, known to the logicians, and loosely affixes two to the forehead of each logician so that each logician can see all the other stamps except those 2 in the Grand Master's pocket and the two on his own forehead. He asks them in turn if they know the colors of their own stamps:
A: "No."
B: "No."
C: "No."
A: "No."
B: "Yes."
What color stamps does B have?

This old topic is locked since it was answered many times. You can check solution in the Spoiler below.
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#2 novellabelle

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Posted 24 April 2007 - 08:03 AM

There is no specification of the two stamps distributed to each person being the same color.
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#3 PookaDot

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Posted 02 May 2007 - 07:14 PM

That doesn't have to stipulated. If you draw out all the possibilities, this is the only stamp distribution in which one of the three would know without all three knowing. It takes a bit but it's worth it. :)
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#4 DarkSpin21

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Posted 06 June 2007 - 04:50 AM

or you can think that the answer would not be able to be determined between red-red and green-green therefore must be red-green
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#5 coolastro1016

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Posted 12 June 2007 - 07:26 PM

she can as long as the other 2 ladies had 2 of the same colors on there heads and if they had the same colors. So if Lady a and c both had double red then b could say for sure that she had 2 green because all 4 red would be exposed. This is the only way because of the person holding 2 in his hand. If the 2 other ladies have red-Green that leaves 2 left of each color and so the hand can have green green or red red or red green but it's impossable to tell. You would need to know whats in the hand to answer.

we need to know the stamps on the other 2 ladies heads.
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#6 anuraag

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Posted 30 June 2007 - 09:32 PM

the only possibility is red and green on each
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#7 bkaps44

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Posted 12 July 2007 - 02:07 PM

we don't need to know the stamps on the other ladies heads. the key here is that it took 'B' a second turn to realize the answer. if both other ladies had the same color, she would've known the answer immediately. since she was able to deduce the colors of the stamps on her head, this is the only situation where she could've done so.

A goes first (with either red-red or green-green) - sees a situation she cannot solve.

B goes next, sees A (with RR or GG) and MUST then see C (with the opposite of A). B cannot know for sure her own stamps.

C goes, sees A (RR or GG) and B (unknown). Since C does not answer, we can infer that B does not have the same color as A. B realizes this, and now knows that since neither A nor C stated their own colors, B must have one of each. B then answers the next time around.
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#8 dsu

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Posted 19 July 2007 - 11:29 PM

Only for those who haven't figured it out completely yet:

If two or three persons have one of each color (RG), then nobody can say one's own color in any number of tries.

If two people have both same-colored stamps on their head (RR RR or GG GG), the third will have the answer instantly.

Only if one has same-colored and another the opposite but same-colored stamps (RR and GG) on their head can the third person (with different colored or RG) have the answer on her second try.
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#9 Slick_Rick9009

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Posted 26 July 2007 - 08:51 AM

Only for those who haven't figured it out completely yet:

If two or three persons have one of each color (RG), then nobody can say one's own color in any number of tries.

If two people have both same-colored stamps on their head (RR RR or GG GG), the third will have the answer instantly.

Only if one has same-colored and another the opposite but same-colored stamps (RR and GG) on their head can the third person (with different colored or RG) have the answer on her second try.




What you said is exaactly how we know that that isn't the case. She DID know the answer and that is how you figure out how she knew.
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#10 Stickguy999

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Posted 27 July 2007 - 09:09 AM

...umm what? didnt quite understand that one
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