For a lens the relation between object distance, image distance and the focal length is given by 1/Do + 1/Di = 1/f, where Do is the object distance, Di is the image distance and f is the focal length. For a converging lens the focal length is taken to be positive.

As the focal length of the converging lens is 10 cm and the the object is placed at a distance of 20 cm in front of the lens, Do = 20 and f = 10. Using the relation provided earlier:

1/20 + 1/Di = 1/10

=> 1/Di = 1/10 - 1/20 = 1/20

=> Di = 20 cm

The image is formed at a distance of 20 cm from the lens on the side opposite to that where the object is placed.