The distance between an object and a screen is 100cm. A lens can be produce real image of the object on the screen for two different positions between the screen and the object. The distance between these two positions is 40cm. if the power of the lens is close to where N is an integer, the value of N is_______-
Option: 1 476
Option: 2 320
Option: 3 570
Option: 4 678