I'm working on this propositional logic question and I did not understand the book answer at all. The book says the hostess knows to bring back two drinks for the first two professors.

When three professors are seated in a restaurant, the hostess ask them, "Does everyone want coffee?" The first professor says, "I do not know." The second professor then says, "I do not know." Finally, the third professor says, "No, not everyone wants coffee." The hostess comes back and gives coffee to the professors who want it. How did she figure out who wanted coffee?

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    $\begingroup$ What are you thoughts? You have to have some initial thoughts on how to approach this. $\endgroup$ Jun 6, 2015 at 4:33
  • $\begingroup$ @MikePierce Are the best approaches when dealing with these problems just to stick with the truth tables. $\endgroup$ Jun 6, 2015 at 4:36
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    $\begingroup$ Truth tables are a bit brutish (it'll work, but it will take a while). For this problem, I would start to consider the different cases in your head. Think to yourself, "Suppose that the first professor doesn't want coffee. What can he say to the waitress? What can he say if he does want coffee? ...". This should be insightful. $\endgroup$ Jun 6, 2015 at 4:41
  • $\begingroup$ @user1238097 I'd like to see a truth table for this scenario... $\endgroup$
    – Jared
    Jun 6, 2015 at 5:00

1 Answer 1


Suppose that the first professor did not want coffee. Is it still possible that everyone wants coffee?

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    $\begingroup$ This is a better answer than the lengthy explanation I was going to type up. $\endgroup$
    – Eli Rose
    Jun 6, 2015 at 4:31
  • $\begingroup$ @Gregory I see what you mean. $\endgroup$ Jun 6, 2015 at 4:34

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