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 offended¹ to learn that by the time you receive this letter that I will be at the Grand Hotel with my 18 year old teaching² 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 mathematician³ 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!