Here is the story: There are totally 20 balls. Only one ball is red and the rest of 19 balls are blue. Everybody wants a red one. 20 persons take turn to draw from the box. Point 1: the probability of drawing a read one has nothing to do with the order. It means the first person or the last person has the equal chance (1/20) to get it. Point 2: the first person has 1/20 chance; the second person has 1/19 if the first is blue; the third person has 1/18 if the second is still blue... and so on. Point 3: the first person has bigger chance to get a red one. For me, I definitely agree to point 1 and I know it is an ancient math problem. Anybody tell me the link or where it is from. I was arguing with my coworkers for the whole day for nothing. They laughed at me for my poor math. Otherwise, tell me my mistake. Thank you for your time and sorry for my poor language.