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 $D$ is given by $m = 1 + \dfrac{D}{f}$.

Given $m = 3.0$ and $D = 25 \text{ cm}$, we have:

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

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

The magnifying power when the image is formed at infinity is given by $m_{\infty} = \dfrac{D}{f}$.

Substituting the value of $\dfrac{25}{f}$, we get:

$m_{\infty} = 2.0$