A somewhat elderly patient was seated in the medical doctor's office and was in progress of being interrogated by an exceedingly stern nurse. The battle of wills was about to begin:
Nurse: How old are you?
Patient: Can you keep a secret?
Nurse: Yes.
Patient: So can I..
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.