Five people are lying, and one of them is telling the truth. How did you work out this man's number? The numbers are 12345.

If A tells the truth = only one person lies, then B, C, D and E all lie, that is to say, all four people lie, which is contradictory to A, so it doesn't hold.

B, c, the same thing. If E is telling the truth, it is even more contradictory for him to say that he is lying.

If only D tells the truth, then four people are lying, which just shows that A, B, C and E are lying.