How old are you?

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.