Two identical, thin, plano-convex lenses with radii of curvature of 15 cm are situated wit...

Two identical, thin, plano-convex lenses with radii of curvature of 15 cm are situated with their curved surfaces in contact at their centers. The intervening space is filled with oil of refractive index 1.65. The index of the glass is 1.50. Determine the focal length of the combination. (Hint: Think of the oil layer as an intermediate thin lens.)

Figure 1