A professor of mathematics sent a fax to his wife:

Dear Wife:
You must realize that you are 54-years-old, and I have certain needs
which you are no longer able to satisfy. I am otherwise happy with
you as a wife, and I sincerely hope you will not be hurt or offended
to learn that by the time you receive this letter, I will be at the
Grand Hotel with my 18-year-old teaching assistant. I'll be home before midnight.
-Your Husband

When he arrived at the hotel, there was a faxed letter waiting for him
that read as follows:

Dear Husband:
You, too, are 54-years-old and by the time you receive this letter, I
will be at the Breakwater Hotel with the 18-year-old pool boy.
Being the brilliant mathematician that you are, you can easily
appreciate the fact that 18 goes into 54 a lot more times than 54 goes
into 18. Don't wait up.