"Mathematic Love" joke

A very logical and somewhat cold calculating professor of mathematics sent this fax to his wife:
Dear Wife:
You must realize that now 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,
Professor Malone
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When he arrived at the hotel, there was a faxed letter waiting for him that read as follows:
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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 than 54 goes into 18.
Your Wife,