Two thin lenses of power 2D and -4D are kept in contact. What is the focal length of the combination in cm? (write the answer only)

Fill in the blank spaces with appropriate answers: Two lenses of powers +4D and -4D are joined together. The focal length of the combination is ____.

A convex lens of focal length f_1 and a concave lens of focal length f_2 are placed in contact. The focal length of the combination is f. Then

Two lenses of power 10D and -5D are placed in contact. Where should an object be held from the lens, so as to obtain a virtual image of magnification?