Three people sit on the floor facing each other. All of them have a number written on their forehead, but no one knows his own number. They all know that none of them share the same number and that all numbers are larger than zero. Furthermore, one of the numbers equals the sum of the other two.

Person C can see Person A has the number “20” on his forehead and that Person B sports the number “30”. The gamemaster asks Person C if he knows his own number, who has to admit he doesn't know. Then Person A is asked about his number but he also has no clue at all. When Person B is asked and admits to not knowing either, Person C gets up and correctly tells the gamemaster his own number. Why? And which number is it?