A concave-convex lens of refractive index \( 1.5 \) and the radii of curvature of its surfaces are \( 30 \, \text{cm} \) and \( 20 \, \text{cm} \), respectively. The concave surface is upwards and is filled with a liquid of refractive index \( 1.3 \). The focal length of the liquid-glass combination will be:
