“What happened to your girlfriend, that really cute maintenance woman?”
“She no longer is my girlfriend. I caught her cheating on me.”
“I don’t believe that she cheated on you!”
“Well, a couple of nights ago I called her on the phone, and she told me it was Sexauer.”
“What happened to your girlfriend, that really cute farm girl?”
“She no longer is my girlfriend. I caught her cheating on me.”
“I don’t believe that she cheated on you!”
"Well, a couple of nights ago I called her on the phone, and she told me that she was choking a chicken.
I had a couple in mind, that I haven’t quite worked out yet. One involves laying pipe, and the other one is about a pile driver. 
“What happened to your girlfriend, that really cute pawn shop girl?”
“She no longer is my girlfriend. I caught her cheating on me.”
“I don’t believe that she cheated on you!”
“Well, a couple of nights ago I called her on the phone, and she told me that she was selling a rusty trombone.”
Am I the only one who thinks the original joke made a lot more sense and was funnier than most of the attempts to “fix” it?
They wouldn’t. That’s how the joke “works”. The mathematician girl says “three unknowns” assuming that the mathematical meaning would be clear, because in her mind it’s obviously a maths thing. The boyfriend, who doesn’t know anything about maths, hears that and assumes she meant random people she’d pick up in a bar or something. Hilarity ensues.
Those would be strangers, not unknowns. The only context I can think of in which you would refer to a human being as an “unknown” is if they are would-be entertainers who aren’t famous. (Or if you want to know how does it *feeeeel *to be on your own with no direction home.) That’s why the joke *doesn’t *work, because there’s no sensible rationale for the boyfriend to mistake the word “unknown” for a reference to sex partners.
Did you ever have a difficult problem or conundrum (not necessarily a math problem, any kind of problem) that you spent a long time pondering but couldn’t come up with a solution, and it got stuck in your head, and even after you went to bed, your brain kept working it over? And it kinda made you crazy that you couldn’t stop thinking about it, and couldn’t solve it? That kind of solving math problems in bed.
There’s not a single, well-known example. Rather, in math, solving a problem with three unknowns is not straightforward like solving for two unknowns. Thus, it invovles mental wrestling.
Mathematically, working with “3 variables” would be equivalent, but the double entendre comes from “unknowns” meaning “variables” and also meaning “strangers”. Yeah, it’s weak. Nobody really talks about other people as “unknowns”.
“What happened to your girlfriend, that really cute prostitute?”
“She is no longer my girlfriend. I caught her cheating on me.”
Badump tish.
Well you know where I stand on this. 
I just didn’t think the original was funny at all. Besides this is kind of fun. 
Trombones rust?
What happened to your girlfriend, that really cute hooker?"
“She no longer is my girlfriend. She’s too kinky for me.”
“I don’t believe that she’s to kinky for you!”
“Well, a couple of nights ago I called her on the phone, and she told me that she was screwing in a a light bulb.”
Meh.
I thought it was funny.
The girl was a math student and when she said she was “in bed wrestling with 3 unknowns” I could picture her up late sitting in bed stuck on a difficult math problem with books, papers, etc. spread across the bed.
On the other end of the line is the boyfriend shocked finding out she is fooling around (wrestling) with 3 unknowns. Unknowns being random guys she met somewhere and didn’t even bother getting their names:eek:, and there’s 3 of them:eek:. What a slut.
The girlfriend is a student. Students drag their books into bed to study because sitting at their desk in their dorm room is uncomfortable. Were you never a student?
For most of my life.
Check out ‘rusty trombone’ on urban dictionary. As with many jokes, the situation may not be literally possible.
Well!
I, for one, am shocked and appalled. Those '50s kids! The malt shop, indeed!
. . . assume a perfectly spherical libertine . . .
Sexauer? We don’t even get a coffee break!
Moved Cafe Society --> MPSIMS.