Three logicians went to bar for having a drink.
The bar person asks the logicians "Do they all want to have the drink?".
The first logician replied, " I don't know".
The second logician replied, " I don't know".
What is the reply of the last logician?
Solution:
Yes (if we want to drink), No (if don't)
Explanation:
Note: The question barman asked if all want to have the drink.
The first logician replied, " I don't know" implies he want to have the drink but is not sure if other two want to have drink or not.
The second logician replied, " I don't know" implies he want to have the drink but is not sure if other third one want to have the drink or not (he knows 1st want to have drink else he would have said 'no')
Similarly, third knows that other two wanna have the drink and hence his decision is decisive.