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. 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."