The maths affair

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 and 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."

When 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 maths teacher at our local college.
I would like to inform you that while you read this, I will be at the  Hotel Fiesta with Michael, one of my students, who is also the assistant tennis coach. He is young, virile, and like your secretary, is 18 years old. As a successful businessman who has an excellent knowledge of Maths, you will understand that we are in the same  situation, although with one small difference; 18 goes into 54 a lot more times than 54 goes into 18. Therefore, I will not be home until sometime tomorrow."