The magnifying power of a simple microscope is (when the final image is formed at $D=25 \mathrm{cm}$ from the eye)

When the final image is formed at $D=25 \mathrm{cm}$ from eye.
In this situation,$v=-D$ from lens formula,
$\frac{1}{v}-\frac{1}{u}=\frac{1}{f}$
we have $\frac{1}{-D}-\frac{1}{-u}=\frac{1}{f}$
$$\begin{gathered} i.e. \frac{D}{u}=1+\frac{D}{f} \\ \mathrm{So}, MP=\frac{D}{u}=\left(1+\frac{D}{f}\right) \end{gathered}$$