The image produced by a concave mirror is one-quarter the size of object. If the object is moved 5 cm closer to the mirror, the image will only be half the size of the object. The focal length of mirror is

In first case, if object distance is x, image distance = x/4. While it is 2nd case, object distance becomes (x - 5 cm) and image distance (x - 5 cm)/2. Using mirror formula, we get.In first case, 1f=-5xIn second case, 1f=-3x-5Solving we get, x = 12.5 cm and f = - 2.5 cm and hence |f| = 2.5 cm