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Persons A and B are standing on a board of uniform linear density that is balanced on two supports, as shown in the figure. What is the maximum distance x from the right end of the board at which person A can stand without tipping the board? Treat persons A and B as point masses. The mass of person B is twice that of person A, and the mass of the board is half that of person A. Give your answer in terms of L, the length of the board.