What is a thin lens ?

A small object is placed $50 \mathrm{cm}$ to the left of thin convex lens of focal length $30 \mathrm{cm}$. A convex spherical mirror of radius of curvature $100 \mathrm{cm}$ is placed to the right of the lens at a distance of $50 \mathrm{cm}$. The mirror is tilted such that the axis of the mirror is at an angle $\theta =30°$ to the axis of the lens, as shown in the figure. If the origin of the coordinate system is taken to be at the centre of the lens, the coordinates (in $\mathrm{cm}$) of the point $(x, y)$ at which the image is formed are:

When a drop of water is introduced between the glass plate and plano convex lens in Newton's rings system, the ring system:

Two thin lenses are in contact with each other. One of these is a concave lens of focal length $20 \mathrm{cm}$ and the other is a convex lens of the same focal length. Determine the focal length of the combination and state how it behaves.

The word $"\mathrm{KVPY}"$is written on board and viewed through different lenses such that board is at a distance beyond the focal length of the lens.

Ignoring magnification effects, consider the following statements.
$\left(\mathrm{I}\right)$ Image $\left(\mathrm{i}\right)$ has been viewed from the planar side of a plano-convex lens and image
$\left(\mathrm{ii}\right)$ from the convex side of a plano-convex lens.
$\left(\mathrm{II}\right)$ Image $\left(\mathrm{i}\right)$ has been viewed from the concave side of a plano-concave lens and image $\left(\mathrm{ii}\right)$ from the convex side of a plano-convex lens.
$\left(\mathrm{III}\right)$ Image $\left(\mathrm{i}\right)$ has been viewed from the concave side of a plano-concave lens and image $\left(\mathrm{ii}\right)$ from the planar side of a plano-convex lens. $\left(\mathrm{IV}\right)$ Image$\left(\mathrm{i}\right)$ has been viewed from the planar side of a plano-concave lens and image $\left(\mathrm{ii}\right)$ from the convex side of a plano-convex lens.
Which of the above statements are correct?