When an object is placed at a distance of 36 cm from a convex lens, an image of the same size as the object is formed. What will be the nature of image formed when the object is placed at a distance of:(a) 10 cm from the lens? (b) 20 cm from the lens?

We know that a convex lens forms an image of the same size as that of the object when the object is placed at a distance of $2f$. 

Therefore, $2f=36$, then $f=18cm$.

(a) When the object is placed at a distance of 10 cm from the lens, then here it means the object lies within the focus (between focus,​ $f$, and the optical centre, $C$).

This position of the object results in the formation of a virtual, erect, and magnified image.

(b) When the object is placed at a distance of 20 cm from the lens, then here it means the object lies between $f$ and $2f$.

This position of the object results in the formation of a real, inverted, and magnified image.