Math Joke

A successful, aging businessman leaves the following note on the dining room table for his wife:

Darling,
I know that we have have a wonderfully open and trusting marriage together, and that I am still very much in love with you; however, we have come to a point in our lives as a couple where you as a 54 year old woman cannot satisfy all of my male physical needs. Therefore, I will be spending the remainder of the evening at the Comfort Inn with my 18 year old executive assistant. I am sure that you will appreciate my honesty and forthrightness in this matter. I will be home shortly after midnight.

When he arrived home, he found the following response written on the back of his original note:

My loving husband,
Thank you for note alearting me to your evening's plans. After reading your message, I went ahead and made plans for my evening as well. Since I teach math at the local community college, it was not very difficult for me to find my own 18 year old student to spend the evening with me at the Doubletree. As you, too, are 54 years old and have used math throughout your business career, you will note that we are on essentially a level field with one exception: 18 goes into 54 more times than 54 goes into 18, therefore I will not be home until some time tomorrow night at the earliest.