Two plano concave lenses of glass of refractive index 1.5. have radii of curvature 20cm and 30cm respectively. They are placed in contact with the curved surface towards each other and the space between them is filled with a liquid of refractive index 5//3. The focal length of the combination is

Two plano concave lenses of glass of refractive index 1.5. have radii

1.	Two plano concave lenses of glass of refractive index 1.5. have radii of curvature 20cm and 30cm respectively. They are placed in contact with the curved surface towards each other and the space between them is filled with a liquid of refractive index 5//3. The focal length of the combination is
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