A biconvex lens is used to project a slide on the screen. The slide is $2 \mathrm{cm}$ high and placed at $10 \mathrm{cm}$ from the lens. The image is $18 \mathrm{cm}$ high. What is the focal length of the lens?

Two plano-convex lenses each of focal length $10 \mathrm{cm}$ and refractive index $\frac{3}{2}$ are placed as shown in the figure. In the space left, water $\left(\mathrm{R}.\mathrm{I}.=\frac{4}{3}\right)$ is filled. The whole arrangement is in air. The optical power of the system is (in dioptre):