There are three people in a room without any equipment.
- Each one has a number on his forehead
- Every person knows the numbers the other two have,
- Every person knows that his number is either the sum of the two, or the difference (but higher than 0)
- All numbers are positive and non-decimal
- No two numbers have the same value
The problem is that nobody of them knows what is the number they have on their forehead (but sees the other two numbers).
The first person is asked, what number he has on his head. He replies, "I don't know."
The 2nd person and after that the 3rd person are asked the same question, and they reply the same.
Again, they ask the 1st person, what number does he have, and again, he doesn't know.
But then, the 2nd person is asked again, and he replies, "50"
How did he find out and what are the other two numbers?