A point light source is located on the main optical axis of the diverging lens at a distance a = 30 cm from it. On the screen located on the other side of the lens at a distance l1 = 10 cm from the lens, a light spot is obtained, the illumination of which is E. When the screen is moved to a distance l2 = 40 cm from the lens, the illumination of the light spot becomes equal to E/4. Find the focal length of the lens.

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