NEET UG 2006 — Optics

A convex lens and a concave lens, each having same focal length of $25 \mathrm{~cm}$, are put in contact to form a combination of lenses. The power in diopters of the combination is:

A microscope is focussed on a mark on a piece of paper and then a slab of glass of thickness $3 \mathrm{~cm}$ and refractive index 1.5 is placed over the mark. How should the microscope be moved to get the mark in focus again ?