Dear Wife,
You will sure understand that I have certain needs that you with your 54 years can no longer supply. I am very happy with you and I value you as a good wife.
Therefore after reading this fax, 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 perturbed.
I shall be back home before midnight.
Honestly,
Your Husband.X

Reply by the wife:
My Dear Husband,
I received your fax and thank you for your honesty. I would like to take this opportunity to remind you that you are also 54 years old.
At the same time I would like to inform you that while you read this, I will be at the Hotel Fiesta with Michael my tennis coach, who, like your secretary, is also 18-years old.
As a successful businessman with your excellent knowledge of math's, you will understand that we are in a 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 back before lunchtime tomorrow!
Your Dear Wife XX