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Here's a pretty easy one.

A new student (a male) was wandering down the hall of his new school looking for the boys' locker room. He asks a nearby student (who happens to be the school bully) to direct him. The bully says, "See those three doors? It's one of those, but I switched the signs around so they're all wrong! Ha ha!"

The new student looked at the three doors. They were labeled "Boys", "Girls" and "Cafeteria". Just then another student walked out of one of the doors.

The new student told the bully "Thanks" and proceeded to the correct door. How did he do it?

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    $\begingroup$ possible duplicate of A modified version of apples and oranges logic $\endgroup$ – generalcrispy Oct 20 '14 at 18:55
  • $\begingroup$ Yes, clearly based on the same principle. It's tough to look up questions stated in different terms but based on the same logic. $\endgroup$ – Matt Malone Oct 20 '14 at 19:00
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    $\begingroup$ Yes, also I think we're going to suffer from the issue of running out of "original" little puzzling logic problems soon (if we haven't already), especially since many question are good and cover the broad general cases. $\endgroup$ – d'alar'cop Oct 20 '14 at 19:09
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    $\begingroup$ To be honest, I think that's a good thing. (Then people will hopefully come up with actual original puzzles, as some have been doing.) $\endgroup$ – Joe Z. Oct 21 '14 at 0:47
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He saw a male student walk out of the door marked "Boys" - in this case, he either walked out of the Cafeteria or the Girls' bathroom. He must've exited the Cafeteria (because he cannot use the Girls' locker room). So one of the remaining doors must be the Girl's bathroom, but it can't be marked as such, so he picks the one marked "Girls".

Indeed, in general, the only case where one cannot deduce is when a male walks out a door marked "Girls" or when a female walks out a door marked "Boys".

If he sees a female exit a door marked "Girls", then she certainly was in the Cafeteria. This means he chooses the door marked "Cafeteria" (as opposed to the remaining door marked "Boys").

If he sees a female exit a door marked "Cafeteria", then she certainly was in the Girls' locker room. This means he chooses the door marked "Girls" (as opposed to the remaining door marked "Boys").

If he sees a male exit a door marked "Boys", then he certainly was in the Cafeteria. This means he chooses the door marked "Girls" (as opposed to the remaining door marked "Cafeteria").

If he sees a male exit a door marked "Cafeteria", then he certainly was in the Boys' locker room. This means he chooses the door marked "Cafeteria".

Of course, it's entirely possible he's been trolled by the clever bully into going into the Girls' locker room unintentionally.

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    $\begingroup$ This is right. Alternately, he may have seen a student of either sex emerge from the door marked "Cafeteria". He would have then known whether the that door was the boy's or girls' locker room. Then it's trivial to find out the other two. $\endgroup$ – Matt Malone Oct 20 '14 at 18:57
  • $\begingroup$ The current edit doesn't make sense. If a boy leaves Cafeteria, the Cafeteria is the boys locker room. At least you didn't tell him to go in the boys. $\endgroup$ – kaine Oct 21 '14 at 0:22
  • $\begingroup$ @kaine all that copying and pasting must have confused me a little $\endgroup$ – d'alar'cop Oct 21 '14 at 0:26
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    $\begingroup$ Of course our student doesn't know that the one he sees hasn't also been a victim of the bully and is emerging from the inappropriate locker room... $\endgroup$ – Julia Hayward Oct 21 '14 at 8:45
  • $\begingroup$ @JuliaHayward Indeed, we have made many assumptions about things. Perhaps our gullibility is what makes us so susceptible to bullying $\endgroup$ – d'alar'cop Oct 21 '14 at 9:42
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I have 4 answers:

  • Boy from Boys = Girls
  • Boy from Café = Café
  • Girl from Girls = Café
  • Girl from Café = Girls

Essentially, you have to either have a "gender from gender" or "gender from café" to determine this.

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