A point object is placed at a distance of $60\text{cm}$ from a convex lens of focal length $30\text{cm}$. If a plane mirror were put perpendicular to the principal axis of the lens and at a distance of $40\text{cm}$ from it, the final image would be formed at a distance of:

For the lens: $u=-60\text{cm},f=30\text{cm}$.
$$\frac{1}{v}-\frac{1}{u}=\frac{1}{f}\Rightarrow \frac{1}{v}=\frac{1}{30}+\frac{1}{-60}=\frac{1}{60}\Rightarrow v=60\text{cm}$$
The image $I_1$ would be formed at $60\text{cm}$ behind the lens.The plane mirror is at $40\text{cm}$ from the lens.The object for the mirror is $I_1$, which is $60-40=20\text{cm}$ behind the mirror.A plane mirror forms an image of a virtual object at the same distance in front of it.Final image $I_2$ is $20\text{cm}$ in front of the mirror.Distance from lens = $40-20=20\text{cm}$.Since it's formed by actual rays converging, it's a real image.(Note: Plane mirrors form virtual images of real objects, but real images of virtual objects).