## Wednesday, November 25, 2009

### White House State Dinner

Imagine that you have been invited to a White House State Dinner. Or a fancy formal Thanksgiving Dinner. You observe the exquisite plate setting in front of you and realize that you have no idea with which of the two spoons you are expected to touch the pre-dinner sorbet (palate cleanser).

You are desperately trying not to reveal your ignorance to a very attractive person sitting across the table from you. So, you decide to quickly ask a person to your right or a person to your left, neither of whom have been served the sorbet yet. The problem is that you have been informed that one of the people that is sitting right next to you is a chronic liar. But you do not know which one. What one brief question can you ask from either of your neighbors, that will clarify the spoon approach for you?

Submit your answer on our Family Puzzle Marathon site. Solve three puzzles and get a prize!

Nicolas said...

Assuming both neighbors know which spoon should be used, I would ask my left neighbor: "which spoon would my right neighbor recommend me to pick?", and pick the other one.

Maria said...

Hey, Nicolas - thanks for helping us from the other side of the Atlantic. This is a third puzzle for Nicolas and I will be surprising him with a prize soon.

Now we all know how to manage in a two choice scenario at the State or a family Thanksgiving dinner. You can't just ask one neighbor what he/she recommends. She may be a liar. But, if you ask her, what the the other neighbor would recommend, she will always point to the incorrect answer. If she is truthful and the other neighbor is lying, she will point to his incorrect answer. If she is a liar, and the other neighbor is truthful, he will point correctly, but she will change it to a wrong answer. In either scenario, you should carefully listen to the recommendation, and then pick the other spoon. And don't forget a confident Bon Appetite glance to a stranger across the table from you.

New puzzle early tomorrow.
Happy Thanksgiving to everyone who celebrates!

Giovanni Ciriani said...

Nicolas answer presupposes the neighbor knows the status of the other person. But what if the neighbor has not been informed and doesn't know anything about the other guy?

I have an answer for such a case. You point at one spoon, and ask the neighbor the following: "Is either one of the following two statements true? You are a chronic liar and at the same time this is the wrong spoon, or you are a truth teller and at the same time this is the correct spoon".

If the neighbor replies yes, then the spoon you are pointing at is the correct spoon. If the neighbor replies no, then the correct spoon is the other one.

Maria said...

Very smart, Giovanni.

1) If the person you ask happens to be a liar and you point to a wrong spoon, first part of your statement is true, that makes the whole statement true. Chronic liar changes this answer to "no". You take the other spoon. Correct!

2)If the person you ask happens to be a liar and you point to a correct spoon, both parts of the statement are false and obviously the whole statement is false. Liar changes the answer to "Yes" According to Giovanni, you pick the spoon you are pointing to. Right again.

3)I the person you ask happens to be a truth teller and you point to a wrong spoon, statement is false, answer is "No". You choose the other spoon correctly.

4)Lastly, if the person you ask is a truth teller and you point to a correct spoon, second part of the statement is true, the overall statement is true, answer is "yes", you pick the spoon you are pointing to.

A very sophisticated answer demanding high level of logical reasoning from the White House guests. Do you think the Virginia couple that sneaked through security, uninvited, and partied at the White House all night long, may have been caught if Security asked them: "Is either one of the following statements true? You have not been invited to this event and you are liars, or you have been invited and you are truth tellers."