A professor of mathematics sent a fax to his wife:
You must realize that you are 54 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 18-year-old teaching assistant. I'll be home before midnight.
When he arrived at the hotel, there was a faxed letter waiting for him that read as follows:
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 the mathematician, you will appreciate the fact that 18 goes into 54 more times than 54 goes into 18. Therefore, don't wait up.