A convex lens is used to throw on a screen `10 m` from the lens, a magnified image of an object. If the magnification is to be `19`, find the focal length of the lens.

Explanation

To find the focal length of a convex lens given the image distance and magnification, we can follow these steps:

### Step 1: Understand the given values
We are given:
- Image distance (v) = 10 m (since the image is real, v is positive)
- Magnification (m) = 19 (since the image is magnified and real, we take it as -19)

### Step 2: Use the magnification formula
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