Alternately:
“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 mounting three giant dangling balls."
“What happened to your girlfriend, that really cute train conductor from Ohio?”
“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 taking a Cleveland Steamer”
“What happened to your girlfriend, that really cute math student?”
“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 in bed trying to solve for geodesics on non-Euclidean metrics, and despite being her boyfriend I had no knowledge of any of her interests, so I assumed it was a sex thing and broke up with her.”
(bolding mine)
I am by no means, even close to being very good at mathematics. :smack:
I can do any and all of the stuff that involves ‘real numbers’, but I just never seemed to be able to grasp the concept of “x+y=6x”. 
How do you solve that problem? (JFTR, that’s a rhetorical question. I do not wish for anyone to attempt to ‘fight my ignorance’ by trying to explain advanced mathematics, to me.
My math skills are sufficient for me, and the trades that I regularly
But how can anyone not see the humor in “she’s wrestling with 3 x’s”? 
“Wrestling with 3 'exe’s.” (as in, 3 ex-boyfriends or ex-husbands)
IMHO, that’s the funniest version of that joke, so far. 
I believe we have definitively proven that explaining a joke ruins it.