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I don't get this mathematical joke - can someone explain?

From Wikipedia:

A physicist, a biologist and a mathematician are sitting in a street café watching people entering and leaving the house on the other side of the street. First they see two people entering the house. Time passes. After a while they notice three people leaving the house. The physicist says, "The measurement wasn't accurate." The biologist says, "They must have reproduced." The mathematician says, "If one more person enters the house then it will be empty.

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closed as off-topic by Najib Idrissi, GNUSupporter 8964民主女神 地下教會, Daniel W. Farlow, Laurent Duval, Thomas Klimpel Mar 2 '17 at 21:26

This question appears to be off-topic. The users who voted to close gave this specific reason:

  • "This question is not about mathematics, within the scope defined in the help center." – Najib Idrissi, GNUSupporter 8964民主女神 地下教會, Daniel W. Farlow, Thomas Klimpel
If this question can be reworded to fit the rules in the help center, please edit the question.

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    $\begingroup$ What part don't you get exactly? $\endgroup$ – S.C.B. Mar 2 '17 at 13:05
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    $\begingroup$ Heh. Heh. Negative people counted in a house. Add one more and... now it's empty :-/ $\endgroup$ – Simply Beautiful Art Mar 2 '17 at 13:09
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    $\begingroup$ This is one of my favorite jokes. But when I tell it, I always mention that the three are watching an initially empty house. This explains why they all feel the need to comment on the situation. $\endgroup$ – Jorik Mar 2 '17 at 14:48
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    $\begingroup$ Wiki has a page on math jokes.... there is something beautiful about that. $\endgroup$ – David Grinberg Mar 2 '17 at 14:54
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    $\begingroup$ @Jorik It's also my favourite, but I tell it with an elevator instead of a house. The pro side is that it can actually start empty and produce the observations mentioned. $\endgroup$ – Hagen von Eitzen Mar 2 '17 at 15:09
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Note that there are $2$ people who enter the house at first. Then $3$ people leave the house.

The joke probably lies in the fact that the mathematician assumes that there were initially no people inside the house (The others do too), and so then there are: $$2-3=-1\text{ people}$$ In the house at the moment. Therefore, if one person enters the house, the amount of people in the house is: $$-1+1=0 \text{ people}$$ Thus, the house will be empty.

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    $\begingroup$ Facepalm! ... It's so simple, I'm mightily embarrassed I didn't get it straight away $\endgroup$ – Homunculus Reticulli Mar 2 '17 at 18:56
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I think the point of the punchline is that the mathematician simply solves the math problem as observed, and appears totally unconcerned with the impossibility of having -1 people in a house in the real world.

Sciences like physics and biology are about explaining the real world, but in mathematics explaining the real world is not a requirement.

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    $\begingroup$ Probably the best answer because it actually explains why the joke is funny and not just why what the mathematician says might be considered factually correct. $\endgroup$ – BallpointBen Mar 2 '17 at 17:50
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    $\begingroup$ @BallpointBen - Well, this is a math site, so you might expect people to just solve the math problem and ignore the real world aspects :) $\endgroup$ – psr Mar 2 '17 at 17:54
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From my understanding, it's just a take on stereotypes of physicists, biologists, and mathematicians. Physicists are generally concerned with real-world measurements, and may often attribute issues to experimental error (counting the wrong number of people here). Biologists end up attributing the extra person due to sex (reproduction). The mathematician sees $2-3=-1$ people in the house, so if one more person enters, there will be $-1+1=0$ people.

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input = 2 output = 3 so, the system contains 2-3= -1 people. So he says if another person enters the house would have -1 + 1 = 0 people :)

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