Mathematics?

[guess who]

A husband wrote the following letter for his wife :
To My Wife. You will surely understand that I have needs that you, being 54 yrs old, can no longer satisfy. Im very happy with you & I value you as a 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 a 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 :
My Husband. I got your letter and thanks for your honesty about my being 54 yrs old. I would like to take this opportunity to remind you that you are also 54 yrs old. As you know Im a maths teacher at our local college. I would like to inform you that while you read this I will be at a Hotel also with Michael, one of my students. He is 18 yrs old also. As a businessman who is at excellent maths you should realise'18 goes into 54 a lot more times than 54 goes into 18.
See you tomorrow