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 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, 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."