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. Since you are a mathematician, you will appreciate that 18 goes into 54 more times than 54 goes into 18. Therefore don't wait up.
Your Wife