Nurse: "How old are you?"
Patient: "None of your business."
Nurse: "But the doctor must know your age for his records. Please, just tell me, I'm going to find it out anyway."
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?"
Patient: "Right. And that's exactly the chance of me telling you my age."