A professor of mathematics sent a fax to his wife:
You must realize 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. 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.
When he arrived at the hotel, a faxed letter waited for him:
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.