 An object 5 cm in length is held 25 cm away from a converging lens. Converging lens is equals to convex lens. Focal length is positive. Object distance is negative. Image distance is positive in 5 cases. Negative in 1 case. Which case you know when the object is placed between focus and optical center. So, an object 5 cm in length is held 25 cm away from a focal. F is equals to not known of focal length 10 cm u equal to negative 25 cm. Lens formula is 1 upon f.