Calculate the focal length of a lens used as simple microscope of magnifying power 20. Consider that the final image is formed at the least distance of distinct vision.

Explanation

To calculate the focal length of a lens used as a simple microscope with a magnifying power of 20, we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Magnifying Power**:
The magnifying power (M) of a simple microscope is given by the formula:
\[
M = 1 + \frac{D}{F}
...