lenses Questions

In the figure shown here, what is the equivalent focal length of the combination of lenses? (Assume all layers are thin, $R=20\text{cm}$, outer lenses $\mu =1.6$, inner lens $\mu =1.5$)

Two thin lenses are of same focal lengths ($f$), but one is convex and the other one is concave. When they are placed in contact with each other, the equivalent focal length of the combination will be: