[Jman28]

<font color="white"> A's number is 20
B's number is 30

C knows that he either has 50 or 10.

When A looks at 30 and 50, he could have either 80 or 20. He doesn't know.

Had he seen 30 and 10, he still wouldn't know (40 or 20).

Then B looks at 20 and 50, and doesn't know his number. It could be 70 or 30.

Had he looked instead at 20 and 10, he would know that his number was 30, since he couldn't duplicate the 10. This is how C knows that he couldn't possibly have 10.

Therefore, he must have 50.

Now, A looks at 50 and 30 and knows that he must have 20, because this number allows C to figure out his own, while 80 would not.

B does the same, sorta.

Right? Badly explained?

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