The chairman of the Mathematics Department at the college sent the following email to his wife:
Dear Helen, You know that you are 54 yrs old and are no longer able to satisfy all of my needs. By the time you read this I will be off to meet my 18-year-old intern at the Holiday Inn. I'm sure you understand and agree that my needs are important. I will try to be home before midnight.

Well after reading the message, she immediately sent the following reply:

Dear Mack, Received your email and I do understand needs. Just wanted to remind you that you also are 54 yrs old. By the time you receive this reply, I will be at the Ritz Carlton meeting with our 18-year-old pool boy. I know that as a mathematics professor you will understand that 18 goes into 54 more times than 54 goes into 18. Turn off the lights when you go to bed and don't wait up, I'll be home very late.