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up vote 49 down vote favorite
4

I remember answering this one incorrectly and when the answer was explained to me I was annoyed with myself. Here is your chance, with the original word phrasing:

Jack is looking at Anne, but Anne is looking at George. Jack is married, but George is not. Is a married person looking at an unmarried person?

A) Yes
B) No
C) Cannot be determined

The first with the correct alternative with the correct logical explanation ... (you know the drill).

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3  
So, out of curiosity, how did you answer it, and what was your reasoning? – Dan Henderson 2 days ago
1  
@DanHenderson I answered that it could not be determined just because we do not know if Anne is married or not. According to magazine.utoronto.ca/feature/… this is the same as the majority answer. – Halvard 2 days ago
4  
Wait, this question wasn't asked before on this site?? It's a famous puzzle... – Sid 2 days ago
2  
It's worth noting that this puzzle assumes that a person can only be married or unmarried - that is, that they're complementary states. For instance, if one admits "divorced" as a separate category, then the answer is "C", as Anne may be divorced. – Glen O yesterday
5  
This is getting silly. Stop tryinng to find loopholes. "What if Anne is neither married nor not married?". I say what if Anne is a bird and the concept of married does not apply? What if Jack is jack daniels, Anne is Queen Anne and George is George T. Stagg and the OP just drank all of them and is drunk when writing this question? Everyone understood the idea in this question. No need to try and dissect it. Just enjoy it and move on. Or simply move on. – Marius 14 hours ago
up vote 65 down vote accepted

Answer is

YES.

because,

If Anne is married, she’s looking at George, who is unmarried. If Anne is unmarried, Jack is looking at her.

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45  
It also works for arbitrarily long chains of people, as long as the first is married and the last is not. For any binary string starting with 1 and ending in 0, there must be at least one 10 somewhere. – Clement C. 2 days ago
up vote 20 down vote

Answer:

A) Yes.

Reasoning:

Case 1. Anne is married. Then Anne (married) is looking at George (not married).
Case 2. Anne is not married. Then Jack (married) is looking at Anne (not married).

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1  
In scenario 2 I think you mean anne is unmarried – Beastly Gerbil 2 days ago
    
Yep. That's what I mean. thanks. – Marius 2 days ago
up vote 13 down vote

Answer:

A) Yes

Because:

Say the symbol -> stands for 'is looking at'. So Jack -> Anne -> George. If we replace the names of Jack and George with either married of not married, we get married -> Anne -> not married. Since Anne can be married or not married, the possible cases are married -> married -> not married and married -> not married -> not married. In both possible cases a married person is looking at an unmarried person. So the right answer is A.

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up vote 12 down vote

Answer:

Cannot be determined

Because:

There may be two Anne's, the first may be married and the second not.

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2  
Good point haha :) – Kevin 2 days ago
4  
@Kevin Good if the question were tagged lateral thinking, not so much for logical deduction. – Mike Kellogg 2 days ago
1  
Seems like it ought to be specified that there are only 3 people, no? – Joe 2 days ago
    
Nice one, although this only works if the two Anne's were not connected in a line of "looking at's" (Jack->Anne1->Anne2->George), otherwise you would get a "yes" where Anne1 looks at Anne2. If we don't know if Anne1 may or may not be looking at someone, the "can't be determined" holds. – Megha yesterday
1  
the language strongly suggest that it's the same Anne so you can't assume there are 2. – adhg 14 hours ago
up vote 5 down vote

Proved the answer in coq:

Inductive person : Type :=
  | jack : person
  | anne : person
  | george : person.
Parameter married : person -> bool.
Parameter looking_at : person -> person -> bool.

Goal
  looking_at jack anne = true ->
  looking_at anne george = true ->
  married jack = true ->
  married george = false ->
  exists p q, married p = true /\ married q = false /\ looking_at p q = true.
Proof with auto.
  intros.
  destruct (married anne) eqn:H3.
    (* Anne is married *)
    exists anne, george...
    (* Anne is unmarried *)
    exists jack, anne...
Qed.

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1  
Please check it's all still aligned as you expect. – Jonathan Allan 5 hours ago
1  
@JonathanAllan Ah, nice. Had to escape some *s, but otherwise good. Thanks – Michael Mrozek 1 hour ago
up vote 4 down vote

The answer marked correct is correct to the spirit of the question. But since this is a puzzle, its fair to point out that there are cases where it might not be answerable.

This is a legal question and it requires that consistent laws apply to all participants to be unambiguously answerable. We don't know that's the case here.

Consider this scenario: Jack is in South Africa which recognizes his marriage and is looking at unmarried Anne who is standing a few feet away in Botswana where Jack's marriage is not recognized. In South Africa we have a married Jack looking at an unmarried Anne. As his gaze crosses the border the law changes and it becomes the gaze of an unmarried man on an unmarried woman.

You can keep the distracting gender/fidelity subtext and resolve the legal ambiguity by replacing "is married" with "is wearing a wedding ring".

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3  
I get your point here... but do look at the tags. it says 'logical deduction' not "lateral thinking" – Sid yesterday
1  
But... the definition of what a wedding ring is may vary between South Africa and Botswana, where the two are standing :-p Point is, if you want to nitpick, you're pretty much making it difficult to answer period. Define looking, define person... define is (temporal distortion anyone!?!) Once in a while a little straightforward logic is a beautiful thing (even if I did manage to get it wrong, humbling isn't so bad!) – JeopardyTempest 1 hour ago
up vote 3 down vote

Is the sequence [1,0] a subsequence of [A,B,C]? Where 1=married and 0=not.

[A,B,C] is either [1,1,0] or [1,0,0]. Both contain [1,0]. Yes.

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up vote 1 down vote

Jack(male, married) -> Anne(female, ?) -> George(male, unmarried). Where "->" means "is looking at"; "?" is one marital status among ("married" or "unmarried" or "civil partnership" or "married and then survived the spouse").

Anne is not determined, so it cannot be determined. If it were a Boolean problem, the answer would be "yes". However, real world problems are rarely binary.

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7  
Full marks for political correctness, but none for logic. Whatever other status they might have, everyone is either married or unmarried. Male or female makes no difference. – DJClayworth 2 days ago
1  
Agree with DJClayworth, except I'm not sure it's full marks for political correctness, either. The sexes of Jack, Anne, and George are not given, only their names. – 6005 yesterday
up vote -3 down vote

Answer: Yes. Because: George is the child of Jack and Anne. Married mother is looking at unmarried child.

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8  
This is not a lateral-thinking puzzle. What makes you think George is the child of the other two? – elias 2 days ago

protected by Deusovi♦ yesterday

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