ray optics Questions

Light travels a distance $x$ in time $t_1$ in air and $10x$ in time $t_2$ in another denser medium. What is the critical angle for this medium?

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: