4D X HAPPY POWER

power 4dGiven, Power of convex lens=4D. Placed at 40 cm from a wall. To Find, Distance of a candle. Solution, It is given that ,Power = 4D. Then, P = 1f. So, 4 D = 1f6. A convex lens of power +6 D is placed in contact with a concave lens of power –4D. What will be the nature and focal length of this combination ? (a) convex, 100 cm (b) convex, 50 cm