Three logicians went to bar for having a drink.
The bar person asks the logicians "Do they all want to have scotch?".
The first logician replied, " I dont know".
The second logician replied, " I dont know".

What is the reply of the last logician?

"YES" if he wants a scotch, "NO" if he prefers something else.

The first says "I dont know" because he wants a scotch, but do not know if everybody wants one.
The second says "I dont know" because he wants a scotch, but do not know if the third logician wants one.(he know 1st one wants one, otherwise, he would reply as no)
The last one knows that both wants a scotch and hence can reply as his own preference.


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