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Source: Iranian Ph.D. entrance exam 2016

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Nice puzzle! The answer is

the third one of the four possibilities

because at each stage, to get from one box to the next, you

take the bottom row, flip it, and put it on the top.

Example: getting from the first configuration to the second.

  • Start with

    B B
    W W B
    W B B

  • Flip the bottom row to turn W B B into B B W.

  • Put it on the top to get

    B B W
    B B
    W W B

  • Let the top right ball 'drop' to get the second configuration.

Solution: getting from the fourth configuration to the solution.

  • Start with

    W B
    B W B
    B B W

  • Flip the bottom row to turn B B W into W B B.

  • Put it on the top to get

    W B B
    W B
    B W B

  • Let the top right ball 'drop' to find the solution.

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