A person holds a short-focus lens in his hand at a distance a1 = 55 cm from his eye and looks at a distant building. Through the lens, he sees a reduced inverse image of the building against the background of the building itself, visible with the second (naked) eye - this gives him the opportunity to judge the size of the image visible through the lens. By bringing the lens closer to the eye at a distance a2 = 30 cm, a person finds that the apparent size of the image doubles. Find the focal length of the lens.

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