(i) A screen is placed at a distanceof 100 cm from the object. The image of the object is formed on the screen by a convex lens for two different location of the lens seperated by 20 cm. Calculate the focal length of the lens used.
(ii) A converging lens is kept coaxialy in contact with a diverging lens both the lenses being of equal focal length. What is the focal length of the combination?