An object placed 12.0 m from a lens forms an inverted image which is 20 cm from the lens.
Describe the image formed if the object is 20 cm from the lens?

Light originating a long way from the lens is focused at the focal point. If we assume that 12m is along way away then the image formed will be at the focal point. The focal point of this lens is 20cm.

Light that originates from the focal point emerges parallel and no image is formed. So when the object is at the focal point no image is formed.