While using the displacement method, for determination of focal length of a convex lens, a student placed the object pin at 10 cm mark on the optical bench and the imge pin at 90 cm mark. He was able to obtain the image of the object pin, over the image pin, without any parallax for two different positions of the convex lens. He observed that, in order to obtain these two positions of the lens, he had to move the lens from the first position to the second position through 40 cm. The focal length of the lens is
(1) 10 cm
(2) 15 cm
(3) 20 cm
(4) 40 cm
