"When The Math Professor's Wife Returns Home From Work, She Finds An Envelope On The Living Room Table" joke

When the math professor's wife returns home from work, she finds an envelope on the living room table. She opens it and finds a letter from her husband:

My dearest wife,

We have been married for nearly thirty years, and I still love you as much as on the day I proposed. You must realize, however, that you are now 54 years old and no longer able to satisfy certain needs I still have. I very much hope that you are not hurt to learn that, while you're reading this, I'm in a hotel room with an 18-year-old freshman girl from my calculus class. I'll be home before midnight.

Your husband, who will never stop loving you.

When the professor returns from the hotel shortly before midnight, he also finds an envelope in the living room. He opens it and reads:

My beloved husband,

You may recall that you, too, are 54 years old and no longer able to satisfy certain needs I still have. I thus hope that you are not hurt to learn that, while you're reading this, I am in a hotel room with the 18-year-old pool boy.

Your loving wife.

P. S. As a mathematician, you are certainly aware of the fact that 18 goes into 54 many more times than 54 goes into 18. Therefore, don't stay up and wait for me.