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: RED Dot  ( 3501 )
« : October 19, 2007, 02:48:34 PM From sunithaguru»



RED Dot


Three masters of logic wanted to find out who was the wisest one. So they invited the grand master, who took them into a dark room and said: "I will paint each one of you a red or a blue dot on your forehead. When you walk out and you see at least one red point, raise your hands. The one who says what colour is the dot on his own forehead first, wins." Then he painted only red dots on every one. When they went out everybody had their hands up and after a while one of them said: "I have a red dot on my head."
How could he be so sure?

 ???

 
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« #1 : October 22, 2007, 09:44:33 PM From spazinvader»

I think i got it.
The first condition is that, they should raise their hand if they have seen one or more red dots from others.
Since there are 3 persons, the possibilities are 8(i.e 2 pow 3)
R R R, R R B, R B R, R B B, B R R, B R B, B B R, B B B
Let the persons be A,B,C.
A is seeing the others first after coming out.He is raising his hand.So there must be atleast one with red dot other than A.
B,as soon as seeing A or C,he will also raise his hand.So there must be one other than B with red dot.
C,on the other hand,could see both of them with red dot.So he will also raise his hand,which makes that there must be one other than him with red.

Since all the 3 have risen their hand,it is obvious that there must be atleast 2 or more red dots.So the possibilities are R R R, R R B, R B R, B R R.
In case of a blue,the one with blue colour as well as one with red could have said their colour in a second(Since they are masters of logic,it is pretty obvious, isn't it?)
They wait for some one to say the answer if they have blue dot.
Since no one is answering,the wiser master understands that there is no blue colour dot and answered him with red colour dot.
« #2 : October 25, 2007, 10:33:25 PM From spazinvader»

Sorry.A simple change.
In case of a blue,the one with blue colour as well as one with red could have said their colour in a second(Since they are masters of logic,it is pretty obvious, isn't it?)

I just wants to mean that in case of a blue dot,there will be two of them answering.The two members will be the ones with red dot and not those with blue dot and a red dot.

Sunithaguru,is this answer right or not.Please do reply.
« #3 : October 27, 2007, 12:04:39 PM From jaiswal»

Lets call them A, B & C. A being the person who shouts out the color of his dot.

Now, lets see from A's point of view :

If A had a Blue dot then B and C would have said their colors as Red immediately because B raising his hand would have meant red dot on C and vice-versa. But when they did not say anything, A knew that he also had a Red dot.
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