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http://plustwophysics.com/how-far-an-object-should-be-placed-from-the-pole-of-a-concave-mirror-of-focal-length-20cm-to-form-a-real-image-whose-size-is-15-the-size-of-the-object/
How far an object should be placed from the pole of a concave mirror of focal length 20cm to form a real image whose size is 1/5 the size of the object