There was a good job going in the office... RIDDLE

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There was a good job going in the office and the boss could not decide which of the three candidates should have it, each of them being worthy of it and all of them very bright indeed. So he set them a problem and the one who solved it would get the job. He showed them five discs, three black and two white and said: ‘I’m going to put a disc on the forehead of each of you. You will be able to see the others discs but not your own. There will be no talking. By pure deduction you will have to work out what colour disc you have, and the one who does so gets the job.’ He withheld the two white discs and put a black one on each of them. After a time one of the men stepped forward and successfully claimed the job. How did he figure out that he had a black disc on?