"Fax" joke

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, I will be at that Grand Hotel with my 18 year old teaching assistant. I'll by home before midnight.
Your Husband.

When he arrived at the hotel there was a faxed letter waiting for him.
It 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. Since you are a mathematician, you will appreciate that 18 goes into 54 more times that 54 goes into 18. Therefore don't wait up!
Your Wife