A professor of mathematics sent a fax to his wife. It read:

"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 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."