JEE Advanced 2015 Paper 2

A large spherical mass M is fixed at one position ad two identical point masses m are kept on a line passing through the center of M (see figure).The point masses are connected by a rigid massless rod of length l and this assembly is free to move along the line connecting them.all three masses interact only through their mutual gravitational interaction.when the point nearer to M is at distance r=3lfrom M.the tension in the rod is zero m=k(

M

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).The value of k is