Lucy had four hats, two red and two blue. She showed them to her three friends, then made them stand one behind another and shut their eyes.

The she put a hat on each of their heads. Then she hid behind a wall and put on the last hat. “To keep all the hats, just one of you must tell me what color hat he is wearing. You can open your eyes but you must not turn around and look at the boy behind you,” she said. "Only one person should call out and no one must talk apart from the person calling out the color of their own hat"

Each boy could only see the color of the hat or hats in front of him. How could they be sure to guess the color of their own hat correctly?

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