My age

Nurse: How old are you?
Patient: None of your business.
Nurse: But the doctor must know your age for his records.
Patient: Well, first, multiply twenty by two, then add ten. Got
that?
Nurse: Yes. Fifty.
Patient: All right, now subtract fifty, and tell me, what do you
get?
Nurse: Zero.
Patient: Right. And that's exactly the chance of me telling you
my age.