I heard this the other day on Capital it made me laugh. It went something like this (sorry if it's not perfect but here goes).
A 54 yr old wife receives a Dear John letter (from 54 yr old husband) which goes as follows 'Dear Wife, by the time you read this I shall be at a hotel with my 18 year old girlfriend. I will be home late don't wait up'.
She replies with 'Dear Husband by the time you read this I will be with my 18 year old gym instructor boyfriend and if my maths is correct 18 goes into 54 far more times than 54 goes into 18!!
If anyone can remember it in more detail please write it, it was so funny. R