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?"

Nurse: "Zero."

Patient: "Right. And that's exactly the chance of me telling you my age."