For an object placed at a distance 20 cm from a concave mirror, the image is formed at the same position. What is the focal length of the mirror?

Explanation

To find the focal length of a concave mirror when an object is placed at a distance of 20 cm and the image is formed at the same position, we can follow these steps:

### Step-by-Step Solution:

1. **Identify the Given Values:**
- The object distance (u) is given as 20 cm. Since the object is in front of the mirror, we take this value as negative in the mirror formula. Therefore, \( u = -20 \) cm.
- The image distance (v) is also given as 20 cm. Since the image is formed on the same side as the object for a concave mirror, we take this value as negative as well. Thus, \( v = -20 \) cm.

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