A beam of light rays converges to a point O on x-axis as shown. The angle of convergence is small. A cube of glass of refractive index `mu = 1.5` and side length `40 cm`containing a concentric spherical air cavity of radius 10 cm is to be placed in the path of the converging beam so that the beam emerging from the cube is parallel to x-axis. At what point C on x-axis should the centre of the cube be placed to achieve this? Give the x coordinate of C taking O as origin.

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