JEE Main 2007 — Optics

Two lenses of power $-15 \mathrm{~D}$ and $+5 \mathrm{~D}$ are in contact with each other. The focal length of the combination is

In a Young's double slit experiment the intensity at a point where the path difference is $\frac{\lambda}{6}$ ($\lambda$ being the wavelength of the light used) is $\mathrm{I}$. If $\mathrm{I}_0$ denotes the maximum intensity, $\frac{\mathrm{I}}{\mathrm{I}_0}$ is equal to