A convex lens is placed in contact with a plane mirror. An axial point object at a distance of `20 cm` from this combination, has its image coinciding with itself. What is the focal length of the convex lens ?

Explanation

To solve the problem, we need to analyze the situation where a convex lens is placed in contact with a plane mirror, and an object is placed at a distance of 20 cm from this combination. The image formed coincides with the object itself, indicating that the image distance (v) is equal to the object distance (u) but with a negative sign, since the image is virtual.

### Step-by-Step Solution:

1. **Understanding the Setup**:
- We have a convex lens in contact with a plane mirror.
- An object is placed 20 cm away from the lens-mirror combination.

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