[robokop]

This is probably wrong but...

<font color="white">Since A can't figure out his number, it means b isn't either 0.5c or 2c. (Since if it was, he would know he must be the sum of the two numbers, as his number must be distinct, and positive).

Since B can't figure out his number, it means the same, but for a and c.

So C knows this, and sees the numbers 20 and 30. This means he must be 10 (30-20), 50 (30+20) or -10 (20-30). But he can't be negative. He also can't be 10, as then the guy seeing 10 and 20 would know he was 30 for the reasons outlined above.

Therefore, C knows his number must be 50.</font>