The screen is located at a distance of 21 cm from the hole into which is inserted a lens radius of 5 cm. Lens converging beam of rays falls, resulting in the screen image of a light spot with a radius of 3 cm. It turns out that if the lens is clean, spot radius changes. Find the focal length of the lens.

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Detailed solution shorthand terms, formulas and laws used in the decision, the withdrawal of the calculation formula and the response.
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