When the image is formed at the near point ( 25 cm ) of a normal...

The magnifying power of a simple microscope when the image is formed at the near point is given by the formula:

$m = 1 + \dfrac{D}{f}$

where $m$ is the magnifying power, $D$ is the least distance of distinct vision (near point), and $f$ is the focal length of the lens.

Given:
$m = 3.0$
$D = 25 \text{ cm}$

Substituting the given values into the formula:

$3.0 = 1 + \dfrac{25}{f}$

$2.0 = \dfrac{25}{f}$

$f = \dfrac{25}{2.0} = 12.5 \text{ cm}$

The focal length of the microscope is $12.5 \text{ cm}$.