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Can You Figure Out The Forehead Numbers Problem?

Can you solve the tricky forehead numbers brain teaser? You're in a group of three people, and everyone has a unique number greater than zero on their forehead. One of the numbers is the sum of the other two. You see 30 on one forehead, and 20 on the other. You don't know what is on your forehead, and that next person says he doesn't know what's on his, and the last person also says he does not know what's on his. Once you discover that no one knows what number is on their own forehead, you figure out what is on yours. What number is on your forehead?

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