Mathematical Difference

A husband wrote the following letter for his wife and left it on the dining room table:

"To My Dear Wife. You will surely understand that I have certain needs that you, being 54 years old, can no longer satisfy. I am very happy with you & I value you as a good wife.
Therefore, after reading this letter, I hope that you will not wrongly interpret the fact that I will be spending the evening with my 18 year old secretary at the Comfort Inn Hotel.
Please don't be upset - I shall be home before midnight."

Then the man came home late that night, he found the following letter on the dining room table:

"My Dear Husband. I received your letter and thank you for your honesty about my being 54 years old.
I would like to take this opportunity to remind you that you are also 54 years old.
As you know, I am a math teacher at our local college.
I would like to inform you that while you read this, I will be at the Marriot Hotel with Michael, one of my students.
He is young, virile, and like your secretary, is 18 years old.
As a successful businessman who has an excellent knowledge of Math, you will understand that although it may appear
That we are in the same situation, there is one mathematical difference:
“18 goes into 54 a lot more times than 54 goes into 18."
Therefore, I will not be home until sometime tomorrow."