On a page called 14 jokes only intellectuals will really understand, it lists:

A mathematician is heading to a house party. Just as he is coming up to the house, he notices there are no cars parked nearby, and all the lights in the house turned off.

Just then, he sees two people enter, and shortly afterwards he sees three people leave.

So he walks up and goes into the house, only to find it empty.

Now, I ain't no brainiac, but I could at least spot sources of alleged humor in the other jokes.
But this one eludes me.
The only thing obvious is the "typical word problem" construction and the fact that the house should not strictly be empty with the mathematician in it.

So what is/are the alleged punchline(s) of that joke?

  • $\begingroup$ I'm voting to close this question as off-topic because it has nothing to do with the creation and solving of puzzles. It is simply asking for the explanation behind a joke. $\endgroup$ – GentlePurpleRain Jan 18 '19 at 21:24
  • $\begingroup$ @GentlePurpleRain, solving what mathematical (or other) aspect makes the joke "funny" is the puzzle. $\endgroup$ – Brock Adams Jan 18 '19 at 21:30
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    $\begingroup$ It doesn't qualify as a puzzle by the rules of this site, unfortunately. A puzzle (on this site) must involve some aspect of deduction, and must have one objectively verifiable correct answer. Trivia questions are disallowed because they are not puzzles; explainations of jokes fall under the same umbrella. $\endgroup$ – GentlePurpleRain Jan 18 '19 at 21:34

If two people go into the house, and three people leave, there are -1 people in the house. So, when the mathematician enters the house, there are now 0 people inside - the house is empty. It's not very funny, nor based in reality in any way.

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  • $\begingroup$ There could have been a person in the dark (appearing) house before the 2 entered. But +1 for a possible angle. $\endgroup$ – Brock Adams Jan 18 '19 at 20:39
  • $\begingroup$ On further consideration, I think you may be right that this is what the author intended and he just phrased/constructed it poorly. ... Let's see if a better answer comes along... $\endgroup$ – Brock Adams Jan 18 '19 at 20:44
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    $\begingroup$ This is the joke. Unfortunately, the wording breaks down when considering "only to find it empty" is a perfectly fine thing to say as an observer in the space. This measure of "empty" wouldn't normally take into account the observer, anyway. $\endgroup$ – Ian MacDonald Jan 18 '19 at 21:09
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    $\begingroup$ @IanMacDonald I agree, this particular instance of the joke is indeed very poorly worded. I've heard it better as "A mathematician sees two people enter a house, and three people leave, at which point he remarks 'If one more person goes inside, the house will be empty!'" The fluff about the party and cars and lights in the posted version only confuses things further, as they are not particularly relevant to the joke. $\endgroup$ – Nuclear Hoagie Jan 18 '19 at 21:32

When I first heard this joke, I took the answer to be that there was already one person in the house before the two people enter, then all three leave together, leaving the house empty when the mathematician approaches.

However, the mathematician cannot appreciate that the number of people in the house starts at 1, rather than 0, especially given there is no evidence to support someone being in the house (no cars parked outside, all the lights off).

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