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Focal length of a lens from the magnification and the object-image distance

This type of question, asked 2 times: 30 Jan 2024, 1 Feb 2024. All 2 questions below, oldest first.

Try each one first, then open its answer.

  1. Question 1

    30 Jan 2024

    25
    The distance between object and its two times magnified real image as produced by a convex lens is 45 cm45\text{ }cm. The focal length of the lens used is ____\_\_\_\_ cmcm.

  2. Question 2

    1 Feb 2024

    23
    The distance between object and its 3 times magnified virtual image as produced by a convex lens is 20 cm20\text{ }cm. The focal length of the lens used is ____ cmcm.