A professor of mathematics sent a fax to his wife:

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 that I will be at the Grand Hotel with my 18 year old teaching assistant. I will be home before midnight.

When he arrived at the hotel, there a fax was waiting for him that read as follows:

Dear Husband,

You, too, are 54 years old and by the time you read this, I will be at the Breakwater Hotel with the 18 year old pool boy. Being the brilliant mathematician you are, you can easily appreciate the fact that 18 goes into 54 many more times than 54 goes into 18. Therefore, my love, do not wait up!