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What seems like a very simple puzzle can have you tearing your hair out!
We have a set of four very innocentlooking cubes  each face coloured red, blue, green or white  and they have to be arranged in a row so that all of the four colours appear on the top, front, back and bottom of the line of cubes. (Alternatively, you could stack them.)
Here is a plan and the nets of the cubes so that you can make them yourself and solve the puzzle: (You can choose your own set of 4 colours if you wish.)

Pictorally, we have
Cube 1

Cube 2

Cube 3

Cube 4

In the following link you need to replace the colours we have used in our puzzle. Our RED becomes Yellow, BLUE becomes Green, GREEN becomes Red and WHITE becomes Blue.
In Ivars Peterson's MathTrek you'll find an explanation of this puzzle.