A professor of mathematics sent a fax to his wife:
Dear Wife,
You must realize that you are fifty-four years old, and I have certain needs that you are no longer able to satisfy. I am otherwise happy with you as a wife and sincerely hope that 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 eighteen 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 fifty-four years old, and by the time you receive this letter, I will be at the Breakwater Hotel with the eighteen year-old pool boy. Since you are the mathematician, you will appreciate the fact that eighteen goes into fifty-four more times than fifty-four goes into eighteen. Therefore, don't wait up.
Your Wife